The BRSCC adopted Group 5 regulations for the 1966 season, which allowed much more significant modifications to engines, transmissions, brakes and suspensions. Changes to the body shape were not allowed though. The new regulations brought the Ford Falcon and Hillman Imp to the BSCC.
The Ford Falcon should have dominated the over 2000cc class as it had the same engine as the Mustang but was significantly lighter. John Whitmore showed the car's full potential when he raced an Alan Mann prepared example at Silverstone, but this was Mann's only such entry. Baillie did well with his example, winning the class championship, but Pierpoint's Falcon was unreliable for most of the season, probably due to its supercharged engine.
Team Lotus and the Lotus Cortina dominated the over 1300cc class, though as in 1965, Jim Clark wasn't able to participate in every round and so they couldn't compete for the overall driver's Championship (Peter Arundel came third).
The Group 5 regulations brought about the end of the Mini's domination of the lower capacity classes. In the 1001-1300cc class, the Mini only won one race, but because Anglia wins were split between Mike Young and Chris Craft, John Rhodes' very consistent 2nd placings gave him the class championship.
The up to 1000cc class was dominated by the Anglias and Imps and there was no Mini driver in the top four of the class championship. John Fitzpatrick won the class and overall drivers championship in his Broadspeed prepared Anglia, though he actually tied on points with John Rhodes.