My experience flying in and around UK and UK to Spain several months ago, none of the flights allowed the guitars on (I didn't try buying extra seats). When I was looking for best deal, it seemed Jet2 was the better choice for extra luggage. I pack extra clothes very tightly around the guitar in the hard case so it can't move at all, and put a tough bungie cord around the neck part on the outside of the case to keep it closed. Don't put a capo or anything else in the case because security will make you open and unpack if they see it in xray. The case took some minor scuffs, got wet, had open latches, etc, but the guitar was fine. I was able to check it at the gate or as special baggage each time, which made me feel a bit better, but it gets unloaded the same, onto the cart and belt, either way.
I took 12 flights last year with it. Some on board, some not, none on board in Ireland, UK, and EU. All the planes were too small.
Taylor 320e Baritone
First guitar was a vintage Russian 7-string classical.