After reading all the comments, I'd say play it safe if the weather is iffy and stay at LHR. I don't think they "do" snow tyres in London and you could get tied up in a traffic mess. If it's sunny, go speak to a cabbie and get him to quote for a round trip sightseeing around Windsor for a couple of hours.
Otherwise, spend the time getting over your first longhaul in the CCR and the Spa and relax. Oh and remember, Toronto is still as cold as a witches u know what!