Plenty of options. Crits and road races will all be listed here -
http://www.britishcycling.org.uk/web...dar_future.asp
and most of the races in the London area will be organised by one of two leagues. Websites here -
http://serrl.co.uk/
http://www.surreyleague.co.uk/
How you go about entering them is going to depend on what licence you currently hold (country and level) and since I'm a Londoner I can't help with that. Worth emailing the race organisers though, their contact details should be on the BC website.