Bob, I was having some fun on the last comment
Now to your points - this is my view and not necessarily that of HMG
HMG does have the right to take whatever measures it deems necessary to protect the airspace and the citizens underneath it. This is enforced in EU reg 300 and ICAO annex 13 that accept that more stringent measures may be imposed. The regulation allows for those with an operational need to be on the flight deck so if they are needed to assist in the operation of the craft they are covered.
The responsibility for deciding who is suitably 'vetted' to have access is given to the operator via the accountable manager security and so not the Capt. This is because it is a requirement that undergo DFT approved training at a different level from crew that looks at these issues in a different way. Also they are the one who would potentially fce charges in relation to breaches that could include corporate manslaughter. Do not forget that the locked flight deck door policy was a result of the BA incident to Nairobi when a mentally ill passenger tried to crash the aircraft . As I recall this was pre 9/11.
I can not answer whether a court would overturn this and on what grounds. The reality is that in operational terms it is an inconvenience that would not justify an airline spending out on the legal costs and Your argument would need to demonstrate either a safety case against, an operational case against and an operational reason for its removal. Personally I do not believe you could achieve that. This is why I do not believe BALPA will win the argument on no screening for crew and carriers accept the delays in sorting out the liquids fiasco.