Air Canada
AC used to operate year-round to GLA from YYZ. I used the service fairly often in the 90's when I made business trips to Europe, having a short stopover in GLA to visit family. Loads were generally high, even in business class. In winter the service was less than daily and also served MAN. Aircraft used was a 762, domestic configuration. AC no longer have 762s and the 763 or 330 is just too big, except possibly in summer peak. That's when competition from the charter carriers is greatest. With GSM and OOM gone, the market is currently underserved, but I think AC will be quite happy to provide connections through LHR for the foreseeable future.
Jimbo