In my capacity as a courier (glorified van driver) I have to meet people at the London airports for consular services (visas etc.) , so I see a lot of people coming off aeroplanes. I've made a point of asking lately, if I see someone with a guitar, how they've transported it. The general concensus is that it is easier to get a guitar into the cabin when travelling from west to east - the GB side are worried that people might use the strings as garottes! Most of the people I've asked are pros, and they've booked a seat for the instruments, but the others have all reported hassles - usually demonstating that the case is locked, and handing the key over for the duration of the flight.