Transat has operated to Toronto from CWL in the past as did Zoom before it went bust. Zoom also had a route to Vancouver albeit it was a joint route, I think with Gatwick or it might have been with an Irish airport. I might be confusing Gatwick with the Toronto service because I think some Toronto journeys began there and operated via CWL. I'm sure someone will remember.
The Toronto market doesn't seem what it once was though. Currently only LHR, LGW, MAN, EDI, BHX and GLA operate the route. Looking at 2001 for instance these airports all operated the route then but so did ABZ, BFS, BRS, CWL, EXT and NCL.
Not being an airline insider it's impossible to know if the route would be viable these days. It might attract decent loads but we know that doesn't necessarily mean a viable route.