Your totally right protecting your car is something you should do, Im not one to force my opinion on others at all, I have worked as a car salesman at a main dealers for some years and know every trick in the book but its such a tricky subject, If you ask the dealer not to wash the car then it could be dirty and even the best pro's may struggle to see through the film for paint defects etc or damage if you let the dealer clean it they may make micro scratches to the paint or worse, If its ok with the dealer you can get it detailed before collection by the pro instead but this happened to me on a focus st and there was damage to the bonnet and the whole blame game kicked off.
To me £250 is alot of money and i wouldn't want to see a fellow enthusiast waste there's without all the info as much of what the detailer does you can do at home.
Pete