Do I have to go through customs, recheck my bag, etc. during my Heathrow layover?

Double check that you don’t have any liquid in your carry-on baggage. UK security process is very strict, no exceptions. If there’s any liquid in your bags, you will have an additional delay. It can take from 5 to 10 extra minutes.
They will call the owner of the bag, search for the liquids, pack them in a clear plastic bag and pass the bag through x-rays again

OP here. Just to confirm what others have said, we:

• did not have to claim and recheck bags
• did not have to do anything like checking in again
• did not have to go through customs or immigration
• DID have to go through security

I just wanted to add this answer to note that we had the option to book flights which would have given us a one hour layover in London. We basically would have had no chance of making it if we had such a short layover. Terminal 5 is so big that you have to take a train to get to other areas within the same terminal (if that’s the case, why not just give them different terminal numbers??). It’s really shocking that BA, of all airlines, would let you book an itinerary with a layover short enough that it would essentially be impossible to make the connecting flight.

British Airways provide a lot of information on T5 on their website, including all about Flight Connections. I’d very much suggest you read through that.

To tackle the main bits, assuming you have a single ticket covering both legs of your journey:

• You will not pass through UK customs
• You will not pass through UK immigration
• Your bags will be checked through to your final BA destination, so won’t have to be collected or re-checked. (Check the baggage tags given to you at checkin to ensure they have been tagged all the way through)
• If your starting airport is able, they’ll give you your second boarding pass
• If not, you’ll have to go to the flight connections desk to collect it
• On arrival, you’ll follow the purple Flight Connections signs, and have your boarding pass + passport checked
• You will have to clear security, which is upstairs from flight connections, before you enter the main departures area of the terminal

As long as there are no flight delays on your inbound, 2 hours will be fine. Queues to get your boarding pass checked before security tend not to be too bad, and while security can be shocking at times, we’re talking 30-45 minutes at bad times.

One thing to check when you arrive is what bit of T5 you arrive in, and where you go from. T5 actually has three buildings, linked by an underground shuttle train, T5A (the main bit), T5B and T5C. If you arrive at B or C gates, and depart again from B or C, you can clear flight connections and security there, which is generally a lot quieter. However, if either flight involves the main A gates, you’ll have to do security there, which can have delays at busy times.

I’ve managed to find a CC-BY photo from Terminal 5 Insider which shows what you’d see at T5A:

This is at the far end of the Purple Flight Connections signs in T5A. To your right are the desks, where you’d go to collect your onward boarding pass if you didn’t already have it. To the left is the UK border, which you’d have got to if you’d followed the yellow arrivals signs instead. Straight ahead is where they’ll check your passport and scan your onwarding boarding pass, then it’s up the escalators in the background to get to security. You need to get your boarding pass scanned by 35 minutes before departure (conformance), so with 2 hours you’ll be fine!

As both legs of your trip are on the same ticket, your luggage will be checked through to your final destination (NBO). These days you don’t have much choice in the matter.

Heathrow offers an online international connections guide, but in general, it is set up for “sterile” connections. It is not necessary for you to be admitted to the United Kingdom, and thus not necessary for you to be processed through immigration. When you debark from your transatlantic leg, there should be signs for Flight Connections, Connecting Passengers, International Transit or some such. Follow these signs to stay airside.

You would then pass through a security screening where your carry-on bags will be examined. You will need to be in compliance with the usual rules about liquids, blades, and so on, so if you filled a water bottle in the terminal in New York, be sure it is empty by the time you reach the screening point.

As you are connecting within the same terminal, two hours should be adequate provided your flight arrives on time and without incident. If you would like more reassurance, you can contact BA to see if they would schedule you on a later connection to NBO.