I'm from the UK and I want a single entry Visa, how much will it be for the 4 day service?

If you mean China visa, you have to apply for it from the China visa application service center now. You have to pay 30 pounds for the visa fee to the embassy and another 36 pounds for the service fee to the center.

If you plan to use the express service, you have to an additional fee of 15 pounds to the embassy another 12 pounds for the service fee to the center.