I did this connection this past July. As previously mentioned the outbound (ORD-YYZ-LHR) was pretty easy although did have to clear Canadian Immigration and Customs.
On the inbound (LHR-YYZ-ORD) 2 of our bags never made it on the flight. It took a LONG time for all the bags from the plane to come off. There was also a lot of confusion as to which belt they would come on. We actually had two bags come on two different belts. Needless to say we spend a good hour to hour and a half walking between all the belts and filling out the forms for BA. We luckily had a longer connection so made our flight.
On the inbound try to get a little bit longer connection as you have to clear both Canadian and US customs and to top it off, as I am sure you already know, you also have to rely on the great baggage handling of LHR