me! me! i can help you with this question!! i live in bethnal green and every now and again i see this HUGE guys come down the road (i swear i saw one with what must’ve been 32 inch arms). they all work out at muscleworks which is just off bethnal green road.
my trainer goes down there with his mate whenever they want to do some serious leg or armwork - no women allowed apparently so there are no distractions (disgusting but anyway).
their web address is www.muscleworksgym.co.uk. they are a very well known and highly thought of gym. you’re bound to get any gear you might need down there too but i’d be careful before asking around. apart from them, i know the soho gyms group are pretty good too. they have outlets in camden, covent garden, earls court and clapham. you’ll probably find them on the net. they all have very extensive free weight areas and the staff are brilliant. hope this helps!