I have used cardboard bike boxes with duct tape corners and also have a Carradice soft bag which I tend to pad out a bit. I don't think you can just send a bike anymore without a box. I do make sure it weighs a hefty 30 kg, which is their limit, they can't throw that around.
Beware there is a size limit for the oversized baggage x-ray machine, I'm not sure what they do if its too big to go through that though.
I do take off the rear mech and leave it dangling on the cable and at Stanstead they asked if I had any lube or grease in the box as they will not let that through as it's flammable blah, blah :roll:
I lied and was ok, gets me that they can sell flammable alcohol products on the other side in glass containers that would make a nice Molotov cocktail, if that's your thing.