There is dc4420 which is this evening near Euston and the last Tuesday of the month. They have talks and the people who attend are mostly corporate IT and infosec company people.
First friday of the month is London 2600, which is a bunch of geeks drinking and talking in the Angel pub.
I think thursday is electronic projects evening at South London Makerspace close to Herne Hill station.
There is an OWASP thing for website security and pubstandards which is webdevs and geeks drinking in a pub.