My F56 Cooper SD is just over 15K miles / 9 months old and due its first service. There's a Halliwell Jones dealership in both Chester and Warrington but they look to have horrible reviews.
Depending where you live, I hear that Liverpool is ok(never used myself), however I used to take my JCW's to Blackpool as Sam and Phil on service are the best I have ever encountered though various dealerships.