In reply to bouldery bits:
As others have said, the Strongholds are really good, LCC are all solid, Bethwall the best for some reason. Wouldn't bother with Eustonwall, it's very busy in the evenings and not very big.
Can't believe nobody has mentioned the Castle yet - it's routes and bouldering, but absolutely massive so has more bouldering than half the bouldering only places mentioned so far. Probably one of the more interesting spaces too. Certainly much more to explore...
Mile End? London's oldest wall. Some nice setting and the Monkey Room is great for getting your pump on. Best pub in the world nearby too, live Jazz on a Sunday night.
Rise is nice enough, but I honestly wouldn't bother going if you're just up checking out London bouldering venues (sorry to disagree Euan!). It's my local wall and even I don't go there much, it's quite small and nothing special.
If you're here in the week I'm always up for a bit of an evening boulder session. Weekends are trickier.
London walls most definitely do get busy in the evenings, but then I've no idea how busy walls up north get as I didn't climb when I was a northerner.
Post edited at 13:55