I'm from SW London which means I'm lucky enough to be a part of the London scene which kinda includes the surrounding areas as well making it probably a more extensive hardcore scene than any you'll find in the rest of the country. It's got a pretty good vibe to it, shows are always fairly laid back and people are by and large friendly/open to new bands or whatever. There are a few dickheads but you'll get that anywhere.
In terms of the good bands there's a lot. The most famous ones are bands like Gallows (who're actually kinda famous even in the mainstream sense), Your Demise, Dead Swans and Blackhole. Equally though there are some amazing more underground/upcoming bands...
Dirty Money RIP
More Than Life
...and that's not even scratching the surface.
Most popular venue for shows is probably the Underworld in Camden but they happen all over central London and the outskirts.
As for labels, they're pretty much covered in this video which also has plenty of info about London and the UK's HC scene.