OFGEM is the national regulatory body for gas and electricity suppliers. I would suggest you complained to British Gas itself first though. Utility companies frequently use agents for door to door selling, and as they do not manage these individuals directly, are often unaware of the unethical / illegal tactics they use.
Utility companies can be fined huge amounts for this kind of mis-selling (npower were fined £1.8m at the end of last year), so I expect BG will be very keen to make it up to you, especially if you make it clear your next step will be a complaint to OFGEM...