Back to the
cheapass decathlon I mentioned upthread, I helped a friend buy it for/with her undergraduate daughter. I don't know how much she rides it, but I spoke to her about it a year after she bought it, and she'd used it to get around Canterbury, to Uni and parties and work, and left it outside the entire time, and it was still going fine. Chain was brown with rust, but it hadn't let her down.
You could do worse than buy something like that to get started, and when you know your mind, sell it, give it to charity or keep it as a pub bike and buy the perfect commuter/adventure bike when you work out what that is.
Notes:
- Decathlon tried hard NOT to sell it to her. Officially they said it was "not for daily use" but I reviewed the components, and they were low end, but not crap. I assume the "not for daily use" is so they can sell you a bike that costs twice as much and more.
- Decathlon also sell some lovely flat bar road bikes, which would definitely be worth a look, and come in way under the bizarre £500 threshold your neighbour set.
- Your nearest Decathlon is Wandsworth, which is a smaller one, but they have a good range of bikes. That's were we bought the city bike. You have to test ride in the aisles. But the bikes are good quality, especially for the price.