Curious Pax
With regard to your questions, I don't think that keeping the plane on the ground at Toronto would be of any value to the airline. The first reason is that all flights are made with reference to the met conditions and appropriate fuel is loaded. They'd know they may have to hold, they usually do first thing at EGLL anyway, and wouldn't want to risk a later departure, missing their place in the queue and having to hold on the ground for an hour to get a stand. The second reason is that it looks bad on the company's figures if a plane departs later than scheduled, even if it is for an operational reason it sill counts as a delay and we all know how BA hate delays...
With regard to the off pier parking at T4, as a controller at EGLL, the T4 situation every morning is diabolical. I'm not apportioning blame here i'm merely saying that there are too many planes and too few stands. The spare piered stands are obviously used up first, remembering that some of the stands have fuelled outbound a/c on them (not allowed to depart before 6am), so these are at a premium. I'm sure off pier and bussing is a chore for the pax, but it's a lot better than sitting in block 93 or 33 for an hour waiting for T11. Believe you me, it's been done...
I'm sure that the ground staff could elaborate far more than me, but I hope that this goes some way to answering your queries.