Hello Matey,
If you're in Fulham may I suggest Denham or Elstree? However I'm not so sure how friendly Elstree is. When I'm in London I live in Hammersmith and find Denham easy to get to and it's the right side of London for you. Also there is Fairoaks or High Wycombe but you'd need a car for Wycombe?
On the age thing and when to start, I wish I'd started at 16 rather than 36 as by now I could be a jaded old pilot, hopefully a captain and wishing I had done something else for a career. Instead I did another career and wished I'd flown years ago...
Other ways of doing it cheaply at your age? What about a glider club and then learn some real fundementals of flying. I believe it helps circumvent some of the steps on PPL at a later date...
Muddy Boots