It depends on what work you're needing to get done. My car has full BMW SH and recently needed an oil service and brake fluid change. Ordinarily I would supply my oil to a BMW. However dealers vary in grades of accepted and willingness to use customers oil.
But on my last service, I checked out maindealerdiscount.com and found BMW Rybrook in Warwick (not too bad for me as I use the M40 to Birmingham quite often). They did the oil and brake fluid for about 1/3 less than their list price, which is already cheaper than some other BMW dealers. Very impressed with them. They didn't 'find' anything to fix, took 1.5hrs, with a good waiting area...WIFI, coffee, tv, papers etc. and more importantly the car still has Full BMW SH. I don't think this affects resale price much, but it does help to sell it quicker and buyers are more attracted to a full dealer service history.