wo wo hold on...
oil is NOT extra. (if its part of the service)
The price they have quoted for a service will include the cost of parts. eg OIL, filters and any parts they renew as part of the service.
the cost they quote is made up of the cost to complete the service, eg labour and the parts used.
however, you m ay get a reduction if you bring your own oil. So if they have quoted for a service and you say ill supply the oil, then they may reduce the cost. (some do, some moan)
id recommend calling them and asking what oil they will use and what discount they will give if you supply the oil.
It may be that the oil they use is ok by you and well priced. in which case just let them get on with it. If you are not happy with oil/price then tell them you will supply the oil and get a discount.
But in short, if they say it wil cost £200 to service the car, thats what you wil pay for the service. They wont bill you £200 + the cost of the oil.
The only times you get charged extra is if you request work thats not part of the service, or if they find something wrong. In which case they SHOULD call you to get your authorisation before completing the work.
Its a good idea to confirm that they will do this when you book the car in.