London is a big ole place so find out where your mate will be living and take it from there. No use crawling through traffic from east London to north west London just to have a lesson cancelled due to poor wx etc...!
I had one student who used to do precisely this and it was highly problematic and difficult to plan lessons around due to inevitable traffic disruption. On a 'dodgy wx day' it was ridiculously unworkable because by the time the student had arrived (2 hours after calling for wx report) it was raining again! Obviously, the student had to blame someone for this and you guessed it - it was us!
VFE.