126,7.
As has been stated, in Uk reg aircraft you must get prior authorisation from the airline management first via your own management.
If we let you in knowing that clearly your licence etc was bonafide, it only takes one smart arse in the cabin to report the whole incident to the CAA. This is why when I have pre-authorised jumpseat rides, I make sure they enter the flightdeck before departure. And if they do venture into the cabin, they at least look official. Smart, pass etc.
You have to remember the emotions of the pax, seeing a random geezer just enter the flightdeck mid flight. Not a very pleasant thing for a nervous/suspicious pax.
So rules need to be followed as with the best will in the world, some whistle blower will wreck your career. As in my company it is class as gross misconduct and will attract dismissal!
But send off the emails and provided you have an airside pass it should be no problems to sort.