Monarch
Those were the days. My first time in a cockpit was 1982, Monarch airlines, Luton to Milan.
A fellow classmate and myself asked the air hostess politely and she whisked us off to the flight deck, about mid-flight. We were there for ages, asking all kinds of silly questions. I remember asking the FO what would happen if the undercarriage fell off (I was only 14).
The FO even gave me his map/weather charts. An experience that is still vivid over 30 years later. It was an important part of my aviation education and love of airliners.
I can understaand the new rules, but it is a shame we can no longer have a peek into the flight deck during the flight. It's not the same when the plane is on the tarmac.