i had visited the cockpit many times, in BA, CX, KA, etc.

contrary to the FA service, BA pilot were the most friendly in my experince. (BA FA also quite good, but just too relaxed compared to the the overwelhming CX FA)
you are always given the headphone set to listen to their communication and also the air control conversation.

you usually need to ask int he first few hours when you board the flight. sometimes you should hand them your namecard. and the FA has to ask the pilot for the visit. whether you are invited many depends on the air traffic condition, weather condition, etc. i was once politely told to do it the next time because they have a junior co-pilot in training.
it is also easier if it is longhaul flight and the pilots are bored.

BA generally would be reluctant to let you view the landing, but CX/KA will always let you do so.