Hi op,
My main concern would be getting food deliveries, if their place is so remote, are they easily able to get food deliveries. Are they classed as vulnerable ? If they need to go to the shops then they would put you at risk.
If you stay at home, you would qualify for priority delivery.
Although I imagine the thought of recuperating in nice surroundings might sound appealing .i think staying in someone else home for this length of time could actually be stressful. It’s at least 3 weeks but likely to be far longer and if travel is banned you would be stuck.
If you stay at home then you have chance to do all those little jobs that you never get chance to and take the time to organise your life and home in a way you won’t at your home.
That said, it depends where you live. You say London, do you live in a detached house. If not and especially if you live in a flat then I would go.
Not enough is known yet about how the virus spreads but it is telling that it was spread a lot in Italy and Spain in places where lots of people live very closely together in apartments.
Finally there is the point that they should have isolated for 14 days nit 7 if there is 2 of them as the virus can transfer without symptoms and they could still be carrying.
Sorry no easy answer which I guess is why you posted.
Ignore all the nasty posts here, one journey in a private car is not going to spread the virus.