I think it might depend upon your card. Mine has a 2.5% foreing currency fee, on top of the exchange rate. When I used my card for purchases in GBP it gave me the same rate as Visa gives, minus the 2.5% my bank charges for foreign currency charges.

I looked up my statement when I got home and consistantly the charges in GBP were 2.5% higher than those where I chose DCC and had it charged in CAD.