People flying to Europe may not realize that there is a significant difference between the baggage regulations (FAA) that apply to US-based airlines and those (CAA) that apply to UK and Europe-based airlines. US-based airlines allow: one piece of hand luggage, 45 inches in aggregate dimensions (plus I think a max weight of like 40 or 50lbs) *plus* a "personal item". I haven't flown with a guitar in a long time, but I routinely fly with a laptop backpack and a rolling case sized for carry-on with no trouble on US Airways. "Personal item" is defined as handbag, briefcase, or laptop case, as examples.
UK and Europe-based airlines, however, allow ONE PIECE of carry-on luggage and it must weigh less than 6kg. I think you can still carry on a handbag or laptop in addition, but they're much fussier about its size, and you may find that if you have one of those they make you check any other item you're carrying. They will *definitely* make you check an addition item if it's more than 6kg, and expect them to weigh it.
Just FYI. I always checked the guitar when I toured because I was also carrying a banjo in a soft case -- I could manage one instrument in a hard case and one in soft but not both in hard, too cumbersome. I always wrapped the guitar neck in foam to make sure there was no empty space around it, and secured any loose items inside the case itself. These were the ordinary hardshell/padded cases that came with the guitars; I always found the Calton cases just too heavy for the kind of carrying I had to do (think European railway stations).