Where and how did the allies scrounge enough food for the Soviets AND the British?

Specifically referring to USSR, although it did suffer a blow to its food production, the country was still able to produce at least some food (although it had to implement strict rationing for most citizens). In fact, Germany occupied Soviet territory that produced "only" 38% of USSR's grain. "Feeding the Red Army" wasn't really dependant on the US - in fact, Soviet government used the country's centralized economy to direct most of the food supplies to the army and industrial workers, leaving the rest of civilians to fend off for themselves on measly rations. However, US still provided about 4.5 million tons of food to USSR over the course of the war - in particular highly nutritious meats and fats. That certainly helped as it went directly to the army and left more food available for redistribution to civilians. Also, to correct one of your comments, USSR was exporting grain until late 50s (and even during the post-war famine).

Source used: The Bread of Affliction.

I've modified this answer, originally just directed at Canada, I'm expanding to include the larger commonwealth. You have to remember that Britain did not go to war, the British Empire went to war. The battle of Britain saw their airforce experience well over 100% losses, yet a steady supply of men, crafts, and food from its overseas empires replenished it and by the end of the battle of Britain, the British air force was significantly larger than it began the conflict as.

“As the Battle of Britain ended, the first young pilots, observers and gunners were emerging from the schools of the [British Commonwealth] Air Training Plan in Canada,” says Leslie Roberts, author of There Shall be Wings, “Soon their tide would be in full flood.”

At this point in time the actual British island made up a smaller portion of its vast resources. Wiki: https://en.wikipedia.org/wiki/British_Empire_in_World_War_II

Getting to food specifically.

** Canada first:

The British empire still existed (somewhat) during wwii and was often referred to as the 'far flung british empire'. At the start of the war, Canada supplied the majority of Britains food stuffs and by the end of the war Canada had transformed into an industrial giant, 4th only to Russia, UK, and the US.

Food was considered a weapon of war by Canadians and over 11 million food rationing pamphlets were produced (and may still exist as a symbol of that time) http://www.rootsweb.ancestry.com/~canmil/ww2/home/ration.htm

More than half of Britains food was produced in Canada, over 70% of flour consumed in Britain ultimately saw its production originating in Canada between 1940 and 41.

http://wartimecanada.ca/essay/eating/food-home-front-during-second-world-war

At the heart of the many of the government’s wartime food policies was the need to feed Canada’s overseas allies and soldiers. As Canadians were regularly reminded by propagandists and advertisers alike, food truly was a “weapon of war.” Particularly after the fall of France in June 1940, Canadian food exports provided an essential lifeline to Britain. By the end of the war, it was estimated that Canadian exports accounted for 57 per cent of British wheat and flour consumption – down from its 1941 peak of 77 per cent – as well as 39 per cent of bacon, 15 per cent of eggs, 24 per cent of cheese, and 11 per cent of evaporated milk consumed in Britain. Much of this was achieved through major state intervention on Canadian farms. Between 1940 and 1943, the wheat acreage in the Prairie provinces was reduced by 42 percent through a combination of subsidies, price guarantees, and other controls. Areas sown for agricultural products needed to meet gaps in Canada’s domestic and export requirements like feed grains, on the other hand, increased by 72 percent, flaxseed by 800 percent, and hog marketings by 250 percent over the prewar period.

Britain wasn't the only recipient of Canadian arms and food. Dubbed the Murmansk run, the Canadians were responsible for a great deal of supplies to Russia.

http://www.veterans.gc.ca/eng/remembrance/history/historical-sheets/murmansk

Beginning in the late summer of 1941, a total of 41 Allied convoys sailed to the Soviet ports of Murmansk and Archangel during the war. The Arctic convoys delivered millions of tons of supplies from the United States, Great Britain and Canada, including aircraft, tanks, jeeps, locomotives, flatcars, rifles and machine guns, ammunition, fuel and even boots.

The Canadian lend lease program (dubbed 'mutual aid') saw over 5 billion dollars flow into the UK interest free.

Britain had entered the war with 80,000 military vehicles of all types; however, 75,000 of these British vehicles were left behind in the evacuation at Dunkirk in 1940. Virtually defenceless on the ground, Britain turned to Canada - and particularly the Canadian auto industry - to replace what had been lost. Canada not only replaced these losses, it did much more. Canadian industry produced over 800,000 military transport vehicles, 50,000 tanks, 40,000 field, naval, and anti-aircraft guns, and 1,700,000 small arms. Of the 800,000 military vehicles of all types built in Canada, 168,000 were issued to Canadian forces. Thirty-eight percent of the total Canadian production went to the British. The remainder of the vehicles went to the other Allies. This meant that the Canadian Army ’in the field’ had a ratio of one vehicle for every three soldiers, making it the most mechanized field force in the war. The Bombardier company of Valcourt, Quebec, built over 150 military snowmobiles. General Motors developed a frame for another snowmobile, of which 300 were built. Canadian Pacific Railway constructed 788 Valentine tanks in its Angus shop in Montreal; its engine was built by General Motors. 5,200 tanks had been built at C.P. Angus and Montreal Locomotive Company shops by the end of the war. 2,150 twenty-five pounder "Sexton" self-propelled guns were built by Montreal Locomotive Works. A heavy utility vehicle body was developed in Canada. Four-thousand such vehicles were manufactured by General Motors in Oshawa. This vehicle body could be mounted on a 4x4 chassis and could, with slight modifications, be used as a personnel carrier, ambulance, light wireless, truck or machinery truck.

It delivered 16,418 aircraft to fill Allied orders, chiefly from Britain and the United States, but also for use by the RCAF and BCATP.

Edit:

Trying to narrow down what Canada supplied to Russia vs the UK, but struggling to come up with a number. The Canadians used around 21% of their military output and sent 38% to the UK. That leaves 41% to "the rest of the allies", which I believe is simply the US and Russia at this stage.

*** New Zealand

Data pulled from here http://www.cooksinfo.com/new-zealand-wartime-food

New Zealand filled in many of the gaps. It was already heavily exporting agricultural products to the UK prewar

Prior to World War Two, farming was the most important sector of New Zealand's economy, and brought in 94% of New Zealand's foreign exchange. From 1936 to 1939, New Zealand exported 94% of its cheese, 85% of its butter, and over 50% of its meat. During each of those three years, it produced an average of 164,000 tons of butter, 88,000 tons of cheese, and 470,000 tons of meat. 80% of all exports combined went to the United Kingdom (UK.)

during the war, the long shipping lanes became a headache as the extended distances simply put them in reach of U-Boats.

In 1940, 90% of all exports were being sent to the UK.

The UK government asked New Zealand to emphasize cheese production over butter. Cheese provides higher nutrition, is a substitute for meat, and is less tricky to store and ship. By June of 1940, New Zealand was shipping 100,000 tons of cheese a year to the UK, 20,000 tons over the 1939 figure. That had to be slowed down though in December of 1940 when, owing to German U-Boat activity, the UK was forced to divert when possible shipping to shorter runs such as Canada and America. The longer the shipping run, the greater chances of running into U-Boats.

They took extreme lengths to get everything they could from New Zealand (meat was de-boned prior to shipping for example) and were frequently routed through South America. This still didn't help and New Zealand actually started to stock surplus food (even built a cold storage warehouse network). Once the Pacific front opened up, these supplies went to the Americans instead.

** South Africa and Sub Sahara

Still finding sources here, but South African manufacturing shot right up during the war as well and was the major driving force in the Urbanization of South Africa. Most exports were not of food though...although I can find many references to "Tinned Snoek"...Snoek being a south African fish that was overtly oily and boney, to the point where starving people would leave the nasty product on shelves regardless of need.

And one last edit...The British people were amazingly resourceful during the war and made great use of all available land. They encouraged people digging up their lawns to plant vegetables, encouraged people to have chickens and rabbits at home, and transitioned all usable land to wheat and potatoes (can feed around 20-40x more people like this). The reason Canadian grain exports went from 77% of their wheat consumption to a total of 57% was due to the British growing and eating their own wheat...1941 was the high period of Canadian grain, by 1942 the British had eaten their animals and transitioned to wheat and potato growing.