The UK has had 100% hold baggage screening for years.
I don't know if bags that come in from outside the UK are all screened before being transferred or just the ones that come from countries that do not x-ray all bags. Bags that are flown unaccompanied are x-rayed from 3 directions and it happens all the time, day in, day out.
I have actually been in the same sort of situation though only on a short haul flight. I decided, along with the guy on the ground, that the damage had been done and there was no point turning around and going back.
Would I make the same decision now post 9/11 ?. Well I have always felt that there is no half-way house in these situations. There is either a security threat, in which case you land ASAP and get everyone off, or there is no threat and you carry on as normal.