Originally Posted by tiggerpurrrz
I was told it was about 5.5 hrs from Toronto to London.
It can be as little as this if tailwinds are strong, but that's not that common.
Of course, if you arrive at LHR, what you gain in the air you will inevitably lose while sitting around either waiting for your gate to come free, or for the bus drivers to bother to turn up to drive you to the terminal.