Graham
Become a silver or gold member with British Cycling, you can do it through the website or by phone - this will cover all your insurance worries as well. Then while your on ask them for a racing licence. Total cost will be about £60 for the year.
That allows you to race pretty much anywhere there's a 4th cat classification although if you want to race a lot locally, things will work out cheaper if you also join the Surrey league as you get reductions on entry to all their races. You'll get sent a calendar of events and Londoncyclesport.com runs up to date additions and changes.
Don't panic! You're already faster than most 4ths you'll be racing with so get yourself over to Hillingdon early in March and you'll should pick up a 3rd cat pretty quickly. A few ACCers are doing the same this year so you shouldn't be alone.
Then you can join us with the 1/2/3's on tuesday nights at Hillingdon and get sucked along at 28mph for an hour or get blown out the back at Crystal Palace after five laps or fight like a junkyard dog to stay in the bunch when the scratch group catches you in the Thursday night handicaps. Welcome to a world of pain and suffering!