"So you are telling me that just because some idiot in a PA28 took the runway immediately behind a 747 20 years ago and then got blown over the entire UK ATC system will continue to ban the use of the word "behind" despite what the rest of the world is doing?"
That is like saying that the only reason there is a cover over the evac alarm switch is that some "idiot" pressed it once when they shouldn't have. No, it's there so it can't be pressed accidentally in a momentary lapse of concentration. As has already been said, the phraseology is written to be "fail-safe". It just happens that the UK adapted to a flaw in the system and no-one else did. Changing the word "behind" to the word "after" is not a big deal, costs nothing and could save lives.