I hold an Indian passport and I recently got thrown out of a plane to Spain via Heathrow right before it took off.
They said, I didn't have a transit visa. I was changing planes in the same terminal and the total time I was to spend in Heathrow was 2 hours.
I have a Schengen visa. No, that doesn't work for London. I begged and pleaded for hours at the gate. Nothing. The worst part is, I did the same route for years with no problem. Apparently rules changed recently.
Trust me, It really is awful being that person who gets discriminated based on where you are born.