Unfortunately charlie, if it's a helpful plumber you are looking for you're better off looking elsewhere as they are a rarity on here. Like you said, if you're not part of the secret club then you're on your own. If you're a plumber with a plumbing problem, they'll help you no problem. If you are someone looking to get in to their precious trade, a few on here will bite your head off and give you the most disheartening of responses (god knows what side of the bed this 'psr' bloke woke up on to respond in that manner). They'll claim they're just being honest but more often than not it is just ultimately unencouraging for those that genuinely want to get in to the trade. After all, why would we join up to this forum and ask for advice if we were not serious about it? And is it not the case that forums are created to share knowledge??
p.s I'm also a trainee plumber in london! It's a pretty scary prospect finding work but hey, if you want to do it and you're determined I reckon it's possible. I heard plumbers' merchants was a good place to look/advertise. Also ring up all your local plumbers and find out what they've got going. I might shoot off to canada next year, do some travelling and learn how to plumb at the same time!
Let me know how it goes with these london companies and plumbers merchants mate! Good luck.