If you book a single ticket from Victoria to GLA then you shouldn't have to collect and recheck bags at Pearson; your bags should also be checked through for the BA sectors from YYZ to LHR and LHR to GLA.
You will go through passport control at T5 Flight Connections at LHR. To that end, please let us know how long you have between arrival time and departure time at that very busy airport.
Your flight from YYZ will almost certainly arrive at one of the satellite terminal bulidings, with the flight to GLA departing from the main terminal building. After disembarking, follow the signs to Flight Connections and Arrivals, which will include a short ride on a free transit train.
At Flight Connections you will go through UK passport control (ensure you have a pen on hand for the UK landing cards) and a security check before going through to departures. Your flight to GLA will probably depart from a low numbered A gate.
You will walk through Customs at GLA airport, a 10 second strolll.