I'm with a UK airline and I occasionally travel on the flight deck. I'm in civvies (generally a suit) and I wear my company ID on a lanyard and my journey starts, endures and ends on that flight deck. This is always with an operational need to be there, with senior company approval in writing, and always with the skipper's agreement. I don't fanny about coming in or out of the door, it'll confuse passengers and it adds an extra risk that doesn't need to be there.
The OP's circumstances sound different to this scenario and as pointed out already it'll be subject to the regulations in force in the country of registration, but on an airline bound for the UK, an aircraft shouldn't have casual observers in the flight deck on approach to UK airspace. I won't quote regulation here but it's quite restrictive, in brief if you're not employed by the airline or by a regulatory body and if you don't have an operational reason to be there, then you shouldn't be there.