First step should be to find a club to race at. If you haven't looked already, go to the BRCA website, or look at the map on the Schumacher website - http://www.racing-cars.com/sitepage/uk_rc_clubs.html
Bear in mind that neither of theses lists are kept completely up to date so it is worth talking to local shops and racers, Facebook is where a lot of club activity happens nowadays.
If you want to run indoors it has to be electric, and that's what the majority of racing is now anyway. I don't think any of the electric indoor classes have to be that expensive, I race Mini's, Touring cars and 2wd bugggies personally and the initial cost of each car is quite similar, but the buggies tend to need more tyres and suffer more breakages.