If you already know what you're doing on a boat but want to start racing, best bet is to turn up on a Sunday morning in one of the Hamble marinas (Hamble Point is a good bet) at about 8am, head for the bar and ask around if anyone is looking for crew.
If you want to learn then (assuming you're based in London), Brighton Marina Yacht Club runs something called 'Club Class' on Saturdays where a couple of members' boats from the racing fleet take out learners and teach them the various crewing positions. Check the BMYC website for details - I think a new course may just be starting. From there, its easy with the contacts you'll have made to hop on a boat for Sunday racing. I started race-crewing about 20 years ago at BMYC (when based in London) and found it a good place to start as it wasn't quite as intense or serious as the Hamble Spring Series racing.