If you are capable of following signs it is perfectly simple to transit at LHR. I find that most of the problems arise from people who are unable to read signs in English and don't even know what airline or flight they are travelling on, so end up in the wrong terminal or at the wrong gate.
Well I had in the past a group of Americans arriving on British Airways from Dubai at T5 and they had 2 hour gap to connect with their American Airlines flight with us at T3, they ended rocking up at checkin just as the flight was pushing, so they started complaining that Heathrows to Complex. Other passengers have said a similar thing, just thought I mention it
But you are true saying most people are unable to read english or dont know their flight/terminal/gate but their are a few who do but still get lost/run out of time.