Not sure if it is actually allowed, but I'm my situation I was allowed. Was on an LHR to JFK flight in Y. I'm quite broad shouldered and unfortunately the passenger next to me was also. Trying to get comfortable, let alone eat my meal was a Massive challenge. Asked CC if it was ok to sit in the jump seat at the back. Spent the next 5 hrs there. Even went through some turbulence and was allowed to stay in the jump seat all bucked until descent.