Not to worry! I just turned 14 and I'm in New Zealand.
I'm not sure if any of you guys have them, but here we have these things called inorganic collections, where people put out all their hard rubbish like couches, desks, etc, and of course, bikes! They're only on once a year, but thats my main source of old bikes
I'd say if you're in the UK, just keep an eye out for old bikes on the side of the road, in the rubbish and the dumps, maybe put an ad in the paper?