The comment is that upgradeable "C" for Elites or lower (and sometimes upgradeable "C" for SuperElites which involves using a seat from the J class inventory) on this routing (and perhaps all routings between LHR and the various cities in Canada) are so booked in business class, that it is virtually impossible to get a seat, right now.

Hopefully, this changes with the introduction of the Virgin service, and the continuing improvement of BA service (in business and first classes), and perhaps the introduction of Canada 3000/Royal business class service, so that a greater number of passengers take advantage of those airlines' premium services, allowing for a lower load factor (and therefore a higher likelihood of upgradeability) in the business class cabin in AC planes to and from LHR.