last time i flew out of london i had a very frustrating time with hand luggage weight...
i was so annoyed with them....it was the one time i wasn't flying business class..
they claimed to me that the restriction was for security - so i said that when i'm on business class, no one checks and they said, well that's business class...so i said, then don't try to claim the restriction is for security if on business i can carry it and in economy i can't....
in the end i moved stuff over to my suitcase to bring the weight down...it took me several moves to get it down enough (i was carrying books in my carry on bag)....
and then i was so angry i went into the bookstore that's past security and bought HEAVY books to fill up my carry on....i must have added a ton of weight (5 of these really massive books)....i didn't care the cost, i was just so annoyed with them....