Interesting debate.
I would just like to say that ofcourse I appreciate that there is a ruling for all flights entering UK airspace and UK registerd A/C to have the flightdeck door locked.
But for say on the 737 if you need to go to the dunnies you still have to get out of the flightdeck, the cabin crew brings us teas and coffees, food, etc. Untill we have a galley and toilet in the F/D we will always be vulnerable as someone pointed out earlier on, in the US they have been flying with F/D door locked for as long as I can remember which didn't stop the terrible events of 9/11.