Im not sure whether you have checked the schedule with BA, but flying with them would mean having a lengthy stop in LHR:
0145 / 0635 BAH – LHR
1730 / 1935 LHR – YUL
Albeit, if you are happy to do this then a return Business fare would be £2219. This has availability on many dates throughout August at the moment.
WTP – This would cost £1519 return.
Personally, on the basis that the connections are much smoother, I would also consider this:
0120 / 0640 BAH – FRA GF
1000 / 1140 FRA – YUL AC
1955 / 0855 YUL – FRA AC
1130 / 1830 FRA – BAH GF
Cost is similar at £2295 club return