As it presently stands, nobody except the operating crew are allowed to be in the cockpit of a UK registered airliner - this is a regulation passed down from the UK DfT ( Department of Transport ) and enforced by edicts from the UK CAA.
There are however ( and of course ) some exceptions to this use of flightdeck jumpseats, e.g. it can be used by company staff, ATC people on famil fights, CAA staff doing checks, people with permission of both the company Ops Manager/Director and the Captain - and always requiring the permission of the Captain.
The rules are such that we now have to lock the cockpit door prior to starting the engines, and the door must remain locked at all times ( albeit with some obvious exceptions ), until after the engines are shutdown.
The exceptions to the flightdeck door being locked are when a cabin crew member requires a visit for operational reasons and / or to enter to tend to the needs of the flight crew, e.g. to bring me in a cup of tea or some grub or when I wish to stretch my legs ( in all senses of the phrase ). In the latter instance, prior to me leaving the flight deck, we must summon a member of the cabin crew, they then go through the door opening procedure to gain entry into the cockpit, I then leave it, and they lock the door behind me....... and it’s all done in reverse when I want to come back in again – it’s a right ‘ole rigmarole.
Coming soon, i.e. in the next few months ( as required under the new official Dft & CAA edicts ) will be surveillance cameras that will monitor the gangway area immediately outside and to the sides of the flight deck door with a television monitor installed on the flight deck – which I must admit might actually come in handy to see if there’s any top totty onboard.
So, all in all I would rate your chances of getting a cockpit visit, whilst the aircraft is in flight, as between "some chance and no chance !" – about the best you could hope for is a quick peek during the disembarking process – sorry.
Alternatively why not go down to your local airfield and book a 'trial lesson' for yourself ? That way you’ll certainly get to see out of the front as well as have the thrill of flying the aircraft yourself, and it’s really not that pricey for a quick 30 minutes of aeronautical fun.