Flew ryanair with mine. They only allow 20kg with a bike! Mine came to 37kg with panniers, tent, and other kit id shoved in there. Then told, I was only allowed the bike in the bike bag! Arghh. So had to put everything into a seperate case, check that in seperately. Still tho, the bike on its own in the case weighed 21kg - but they did let it on.
£40 for the bike (one way) £50 for the case + £35 for another case with the other bits in it. Cost more to take my luggage than it cost to take me!
It is a pain in the backside. Ive been offered a free bike service at the shop where I get my bike. but.. i now live in spain, and at £40 each way to book a bike on.. its just not worth it!
If youre not going too far, and can travel by ferry, do it! Its far easier! No dismantling of the bike, no worrying about it getting damaged. I did Caen to Portsmouth. Me, the bike, my luggage, and a room for the night for £99. Cycle on, cycle off. Fantastic!