Obviously this scenario would be completely different if the child lives in London as they may be much more familiar with public transport etc.
How old would you say is ok for a sensible dc to travel by train to London with their friend? They will be there all day sightseeing and then get the train back.
DC has been to a couple of our local large towns on the train with friends and I grew up in a big city and remember being fairly young when I started travelling there with friends.
I think London feels different to other places because it is so big and if you’re not from there or don’t visit often then it can be hard to navigate yourself.