GameDevMap.com provides an updated list of London Games Companies via a search, pop over now via the link to see a full listing, including links to their requisite web pages.
Information available via their database includes type of business (
Developer | 
Publisher | 
Developer & Publisher | 
Organization) and platform (
Mobile / Handheld | 
Online Developer) and includes said information and links to Games Industry companies worldwide, a really fantastic resource!
Many thanks to the awesome folks from GameDevMap.Com for this data!