Tecsos sell onsies 3 for £7, for newborns that are adorable and will probably fit your little one.
Also it's not to hard to make a cloth cone, think there are patterns online. I made one for Pippi, awhile back, but she's not had her op yet so haven't used it with her. It can be made to slip on the collar, if you were worried about them it slipping off. Or of course you can buy them. I think amazon.co. uk sells them also. They used to sell the blow up ones, I expect they still do.
Good luck with the op, let us know how she does. Will be thinking of you. We'll probably be booking Pip in this month also. She also is the sort to chew her at the operation site.
cone
http://www.amazon.co.uk/s/ref=nb_sb_...%3Acone+collar
or pj's
Amazon