London's start-up community is spread over the city with clusters around the Nathan Barley-esque “Silicon Roundabout” (map) and Bermondsey St.. Most of the companies are well networked, and getting to meet a cross-section of the people involved is surprisingly easy with membership to a bunch of groups on meetup and ning.
Saul Klein's OpenCoffee group is probably the best place to start. OpenCoffee meetings are free, weekly meet-up of entrepreneurs, investors and service providers and are a great way to get a feel for what's going on. OpenCoffee meetings are spread around the globe, so if you're not in London the odds are there might be a closer meeting.
Meetup.com has a number of other groups worth checking out, from Bootlaw, providing free legal advice, through the more exclusive DrinkTank and into the punitive beer-sodden monthly shout-fest that is MiniBar.
There are a host of periodic events, some of which are worth checking out. First off, TechCrunch UK's TechCrunch Talk events are usually interesting, and Seedcamp is fascinating for the events and noise it generates. Seedcamp brings together a lot of smart individuals, strong advice and great resources, but the process involves offering up more equity than many might expect for €50,000.
Other events, like Future of Web Apps are worth a look in, although perhaps more out of technical interest than for any networking they might offer, and there are plenty of similar events like Future of Mobile and Internet World.
There are plenty of other investment-focused groups and events, but they're for another post. Feedback on your experiences at these events -- or others -- would be appreciated!