I wondered too, but last time I was in the UK all my charges in GBP converted to CAD at about $1.60-1.61 per £ and the DCC charge for the rental car where I did it in CAD came to $1.57ish. I'm going by memory, but the exchange rate was about 3-4 cents cheaper when I had the merchant charge me in $ rather than £. I had charges in GBP that closely bracketed the CAD charges and were at similar rates so I know it wasn't a currency fluctuation.

This was with a TD Bank visa.