I agree -- technically, you're screwed. But I was ok once on a LHR-IAD-LGA(UA) and LGA-YOW(AC) two separate ticket itin. LGA protected me on the last flight before I even left IAD -- they knew the flight was late getting in and they even let me go to the RCC to relax (these were the days when I had Prestige status).
DutchSE: I have consistently used this method on my trips to BRU when I was studying there and didn't run into any problems. Just book yourself on the 0730 or 0945 BRU-LHR flight and no earlier than the 1300 flight to YYZ and you should be fine.
Mind you also, given the weather at LHR and BRU lately, I wouldn't expect an winter storm delays. The return may be challenging...
I would book the 1640 LHR-BRU flight just to give yourself a bit of leeway if you're afraid of storms and what not.
ButAlso, because BD is a STAR partner, they can check you in for the AC flight, so you will not be in the same perilous situation as if you were flying separate tickets on unrelated carriers. So you should be in a protected situation no matter what the weather turns out to be, as long as you have booked your connections within the legal time limits.
Don't count on that. The effers at BRU are incompetent people. They can check you in, but they probably won't and tell you they can't do that or don't know how to do that or something stupid.