I bought my 1100 from Bracken, and had it serviced there. Last time I went (over a year ago), Paul told me they were stopping doing BMWs - they used to do parallel imports of GSs - to concentrate on being a KTM main dealer. He didn't seem too interested in doing a repair to my bike, so I went elsewhere. Shame, as they were really good/knowledgable etc and did off road prep for the overlanders. They had a ratty old beemer for a 'courtesy' bike. Not sure what they are up to at the mo', so let me know if they have returned to BMs.
I've been using Pat at Mobitec recently for servicing, as like many others here, I live and work in 2 BMW dealer free zones. Pat is not cheap, but wins on coming to you to pick the bike up, and drops it off when complete.
For tyres and basic work to brakes etc, I go to Chas Bikes in SE1 (the couriers friend) when I can't be bothered to DIY.