It depends on the area of the country you live in, the size of the bitch, and also varies from vet to vet within an area. I'd guess an average would be about £150 - your best bet is to ask your vet directly. :)
I have recently had a bitch spayed as an emergency and while the bill came to £307, the cost of the spay alone, but including GA, was £109.86 not incuding VAT. Mine was an American Cocker, and it seemed to go by body weight.