Sorry to say, you won't get a Halo for under £20 and if you find an Egg for less than that it probably will need new shells/board/lid or door which will cost money to replace. Should be able to pick one up for £25-30 though.
Yes i know i wont get a halo, it was a joke really.
But i dont mind if the egg has a couple cracks. I have seen a couple ages ago for £20 odd that were good nick.