Ok the best option would be the mismate jab and it would be very easy at this point. I take it this was accident?
If you do continue to go ahead are you willing to put in the time raising puppies is no easy thing especially if the mother rejects them. Will you be prepared to sit all day and night with them for the first week or so, bottle feed them and help them toilet if need be every couple of hours?
Not to mention the cost vet care for the bitch, £1000 for the c-section should something go wrong with the birth and that can happen with any breed, puppy milk if need be, vet checks, worming and all necessary puppy care. Not to mention taking the puppies back if the new owners can't take care of them and I think there can be up to 10+ puppies in these litters. Not to mention the lost wages from having to stay with said puppies for the first couple of weeks.
If you can do all this then go ahead but personally I would recommend the mismate