I sympathise greatly with all those in your postion , i greatly appreciated a few trips on the J/S many moons ago in my wanabe days . What i can say to you is that for UK airlines , the government tells us what to do and the government at the moment and for the foreseeable future (decades) says that there is no possibility of flight deck access in flight to anyone who is not operating crew.
This includes off-duty colleagues employed by the airline.
There are exemptions in our flight ops manuals for certain special categories of person eg, engineers travelling to AOG aeroplanes . These are subject to the written approval of the Flight OPs Director.
Keep asking at the the door though on the ground , we are aleays pleased to encourage those who are back where we once were .
Good Luck