I got my first BMX for my 30th b'day and spent the winter at indoor skate parks. It was amazing. Next summer, gave racing a go. If you do the Midlands regionals, it probably the cheapest form of bike racing you can get. Few quid to enter, practice, 3 motos and a final, I think. I raced Novice 20" and am proud owner of a little plastic cup for 4th in the series, or something equally unimpressive. Had a blast though. Sheffield, Rotherham, Chesterfield all not too far from you if you're near Manchester. Maybe turn up and watch first to gauge how you'd fit in.
The only thing I found is switching from a DH bike to a BMX was pretty hard! The BMX felt jittery and harsh, then swapping back, the DH bike felt like a boat. I spent a few months feeling rubbish on both bikes.
Another option, if you have a hardtail, is to race MTB class at the regionals. Seems like most rounds have them. Hardpack tracks, usually all weather, great winter fitness.