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All the info on entry for the London League is available on their website;
http://www.londonxleague.co.uk/info/
Basically you have to pay the £10 registration fee for the timing chip and associated admin costs.
I've only ridden Happy Valley once, and it was a pretty unexciting experience. The venue is some playing fields and the course just did lots of hairpin loops around them, with a couple of slightly more challenging sections at either end. It may have changed though, this was five seasons ago I think.
I think you have to pay the affiliation/ chip fee. Which isn't great if you are only doing one race.