I've carried a guitar onboard as cabin baggage from NYC to LON. (son's birthday present purchased in US) . It was in a hard case. It just lay in the overhead locker above my seat, no problem. However this is about 4 years ago so I don't know if the rules are still the same.
I know my son still travels everywhere with his guitar as carry-on. Once (out of a lot of flights) BA made him check it in at the gate. LGW-MRS flight which he had taken a hundred times before with his guitar. Just happened upon a cranky jobsworth at the gate I think.