Hey it seems you are talking not about hand luggage.
It is a real hassle right ? Have you gone through such scenario?
I have other flight option but have to wait long transit in London to avoid Toronto.
Following from your other post re a 16 hr layover in London to be honest taking the Toronto route to connect to a flight to Ottawa will be 100 percent less hassle than staying overnight at London Heathrow where you have to board a bus which is not free to get to a hotel outside the airport perimeter. This could take as much as 30 minutes one way depending how many other hotel stops plus waiting time for the bus. You could of course take a taxi but as with many airports you would need to be prepared for excess cost.
And just to add to above you have the hassle of clearing UK immigration which depending on time of day can take a long time if not an EU citizen.
Whether you enter via Toronto or Ottawa as first POE the immigration formalities will be the same and dropping off your luggage at a connections desk is a simple process given your luggage would usually already be tagged through Toronto to Ottawa. Once you get to Ottawa you arrive on a domestic flight so you just collect your luggage and go. For immigration through Toronto you probably need to allow a minimum of 3 to 4 hours to make the connection.
As always regardless the advice above check with the airline.