There are a lot of tech meetups in London, you could literally go to a different one every night if you wanted to. Finding out about them is slightly trickier as they're all organized in different places (mailing lists, facebook, meetup, etc.)

I've found the best way to find out about them is to follow tech people on twitter.

If your looking for startup events check out this blog post I wrote about the startup scene in london:

http://blog.awesomezombie.com/2010/06/london-startup-event-g...