This seems to be the new YYZ-LGW BA 2270.

As immigration seems to have improved at LGW recently, I'd say it'll probably take you around 45 minutes from touchdown to getting landside, leaving you plenty of time to re-check your bag as both airlines are at the South Terminal. However, even an hour's delay would screw you up badly... how flexible would your onward ticket be, and/or how expensive would it be to lose it?