I have yet to meet a Captain who thinks that the rules over flight deck access are anything other than crackpot equally I have yet to meet a Captain who would disregard the rule without approval from the flight ops director, it may well turn out that the Virgin skipper contacted his/her FOD on HF to request such authority, if so he's in the clear, if not I suspect he/she is in trouble.
I would love to have the PM on the flight deck( this one not the last) but the answer would still be no, the president of the US is the commander in chief of their armed forces and within reason can o pretty much what he wants, the last one did a carrier land as co pilot if I recall
Certain employees may travel on the FD but only on authorised fam: flights
I have herd of a good few flight deck who fly for a certain Irish airline often jump seats when flight is full and trying to get home they also avoids check in and APD by travelling in uniform via crew channel, which in my view breaks at least three rules, FD access , use of airport ID when not on duty, evasion of HMRC APD fee's in other words just like the old days!