from cambridge your closest track may well be Stotfold. The East of England is going through a bit of a quiet spell at the present time, although as has been mentioned there is talk of a revival at Eastrax, which is located on the A17 between Kings Lynn and Holbeach.
Other clubs in the East would include Norfolk buggy Club which is just north of Norwich, MERCC which is near to Chelmsford, and Coastal Model Car Club which is based in Ipswich.
Your best bet maybe to check out the BRCA website,
www.brca.org which has a list of all the clubs in the UK and what classes they race.