From the North East. Was allowed down to London for a week (maybe 5 days?) after GCSEs, aged 16.
Went with a friend, stayed with her Aunt in zone 4. This was about 15 years ago now, so no phone tracking, don't remember having Google Maps on phone.
Conditions from my parents were that I paid for it myself, including spending money. Friends aunt had agreed to feed us, but we did do a couple of cheap lunches out. Mostly wandered around, did museums and bought souvenirs. We got the megabus down and back, so much cheaper than train (it was £8 each each way, so £32 total), bought travel cards in advance, so we wouldn't get stuck anywhere. I'd been to London once earlier in the year, but friend had been down loads.
I'd demonstrated my competence at navigation already, was pretty trustworthy, and was going abroad with Guiding later in the summer (although that was with adult leaders). We also did a list of things we wanted to do, and my parents looked over it before we went, and basically approved it all as safe enough that they were happy. We also were not the kind of 16 year olds to try to go out clubbing at that age. My sister would not have been allowed to go on the same trip at a similar age, due to the fact she would have probably tried to go clubbing, her and her equivalent friend would have (& had track record for) left each other, and she was liable to get lost and ring my parents in a panic (and has done this as an adult too).
I'd say that was pretty reasonable, and if you suspect your son will be similar to me, then at 14 it's fine to let him go, with some similar caveats.
If you're not sure, then it should be a 'this is what we need to see before we agree to let you go to London overnight with your friends'.
And what would be an automatic no, for example if he goes out and doesn't come home on time and doesn't contact you, if he breaks your trust by lying about where he's going, if he isn't saving money towards it as you expect. I'd also say, if you let him go, be clear that it's not a yes for every time in the future, and that it's a bit of a trial.
If it's a no, then it's totally fair to say you'll reevaluate it next year, when he's older and wiser, and that you understand it may be disappointing, but his attitude in accepting your no this time will play into your future decision too.