To all those of you considering transiting thru LHR, please keep in mind, that if your trip is booked on two separate tickets, Air India's liability ends at LHR. You would then have to collect your baggage and check in again at departures. This would require a UK visa. Unless, if Air India guarantees you that your baggage can be checked thru all the way till YYZ, then it is a risk worth taking.
With the current uncertainty, there is also a chance of mandatory 14 days quarantine being applicable at LHR for all international arriving passengers. Although logic dictates that this should not affect passengers in transit, but you never know.