Is travelling with 1 friend permitted in the UK now that the lockdown is enforced?

The morally correct answer is obviously "don't do it".

On the other hand, there is little chance that anything or anyone will stop you, so long as you are following the general social distancing rules. UK policing strategy is generally "consensual" so there are unlikely to be any random "stop and search" checkpoints on your trip.

There is no shortage of fuel in the UK. In fact supermarkets have started a fuel price war, with price cuts of 20% or more.

Aside from the obvious infection risks, the biggest question to consider is what you plan to do during this trip, since only essential businesses (supermarkets, pharmacies, etc) are still open for business. Don't expect to find any "tourist activities" still operating.

Since the question and most of the answers were written, what had been strong recommendations from the government have been turned into regulations with the force of law, in the form of The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020.

Of relevance to this question:

6.—(1) During the emergency period, no person may leave the place where they are living without reasonable excuse.

There then follows a list of reasonable excuses, which includes things like shopping for food, exercise, seeking medical assistance, providing care or assistance, going to work if it's not practical to work from home, and the like.

A trip, presumably for leisure purposes - whether with members of your household, or with one friend, or on your own - is not on the list, so would not be allowed.

Anyone breaking the regulations can be fined. The fines start at £60 (reduced to £30 if paid within 14 days), and then double for each subsequent fine, up to a maximum of £960.

The regulations (at least in their current form) expire on 26 September 2020.

For your information... story dated 22:56 24 March:

Police have been stopping cars to make sure people are only making essential journeys.

It comes after Boris Johnson yesterday put the UK on lockdown, ordering Brits to remain in their homes unless they have a valid reason to go out.

He warned that police would issue fines to those found disobeying the new rules, and today officers have been on the streets making sure drivers do not flout them.

The four reasons you can now leave your home are:

shopping for basic necessities, as infrequently as possible

one form of exercise a day - for example a run, walk, or cycle - alone or with members of your household

any medical need, to provide care or to help a vulnerable person

travelling to and from work, but only where this is absolutely necessary and cannot be done from home

Update 26 March: Police set up checkpoints to stop drivers during coronavirus pandemic

With the current lockdown rules, will I be stopped and fined?

What are the possible consequences?

If you're lucky, you will be stopped, fined, and sent home.

If you're unlucky:

• You get sick from COVID-19.
• You pass on COVID-19 to (many) others, some of whom may die, and more who will have to go to a busy hospital.
• You get fined or even have to go to jail (with the actual sentence possibly postponed until after the pandemic). Currently (2020-03-24) no jail in the UK, but things might move fast; in other countries it's already possible (e.g. in Germany it may be a Straftat to violate the rules in some cases rather than an Ordnungswidrigkeit — I don't know how Straftat or Ordnungswidrigkeit best translate to the English language or UK law, but literally Straftat means something like "punishable act"). That doesn't seem to be the case in the UK currently (BBC reports fines), but who knows how quickly this may change if fines appear ineffective? I am not a lawyer but I would not assume it will remain with just fines if those are shown to be ineffective and the situation is worsening daily.

Do not endanger the lives of others. Do not travel if you can avoid it.