Now is actually a good time to start as, for reasons I've never understood, road race fields seem to decline towards the end of the summer. I think it's because a lot of people start racing very early in the season and then run out of motivation by July.

I don't know whereabouts you are in Sussex but the Surrey League is a great way of getting into racing;

http://www.surreyleague.co.uk/

If you can get into SE London on a Tuesday evening then there is a weekly night of racing at Crystal Palace. The circuit is quite technical and tough but you'll get a feel for it very quickly. It's actually a good place to start as even if you get dropped you'll normally find there is a group of people of similar ability that you can race around with.