Hi! I did a lot research on transit visa requirement for London airport.
Based on that if you hold valid document for entry in canada( USA, Canada, Australia, New Zealand), you don't need to get transit visa.
as long as you don't have to leave airside you don't need one.
remember this... you can change terminal without changing airside or passing through uk immigration check... Maximum changing time is around 20-30 min(you can change terminal by foot or by bus but not by underground tube)..
You will need transit visa if.... 1) you have to cross uk immigration check... 2) you have to change airport 3) you have to go for luggage check at Heathrow airport(make sure that your luggage is checked in for your final destination) 4) stay more than 24 hours( not sure about it)
Hope I could Help!!!
Cheers!!!!!!!!!!!1