Bristol has had a Toronto service in the past at various times but never non-stop so far as I can recall.
For example, around 1990 Odyssey International operated from BRS via NCL with a B 757.
In summer 2000 Royal Air did so via Glasgow with an A310. The airline was then absorbed by Canada 3000 who operated the route in summer 2001 with a B 757, again via GLA, only to go out of business later that year.
The last attempt was in summer 2008 when Globespan operated from BRS via Dublin to its version of Toronto at Hamilton. It was not a success and wasn't brought back for 2009 though Globespan went out of business soon after.
I believe the Exeter route to Toronto is not operating this summer.
Incidentally, the TOM B 767s do fly from BRS to Sanford non-stop nearly all the time, possibly all of the time. Cancun though requires an outbound fuel stop at Manchester.