The LHR-MAN sector is in fact only usually about 30mins airborne - add a busy cabin and a short/non-existent cruise and your chances are slim to non-existent unless you manage to talk to the crew/dispatcher EARLY, giving them a chance to ask the Capt, and leaving enough time for you to meet the crew and strap in the front at least 15 min before pushback.
You would most likely need to stay in the
JS for the duration of the flight, and would probably miss your meal and cup of tea - unless the flight was REALLY quiet. That said, MAN based crews are experts at the short service and I doubt you would find many objections from the flight deck, save for the reasons above. Have fun!
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...proceeding below Decision Height with CAUTION...