Well once last year, I got a very economical deal through Orbitz from Abu Dhabi to Toronto via London. This involved a combination of Etihad & British Airways, so a Terminal change at Heathrow was also required ( terminal 4 to 5). Interestingly no one asked me for a UK visa on any terminal at Heathrow. However, I was asked at Abu Dhabi airport prior to boarding if I had a UK transit visa. I told them I didn't require it as I had a valid UK visit visa since last year and, of-course, that was accepted. Similarly, on my return BA flight from Toronto to London, the lady at counter ensured that I had a UK visa before issuing my boarding pass.
Hope this helps.