Hello sir,I usually use a local garage, they dont advertise as bmw specialist but they have really good knowledge of bmw audi and mercedes.
depending on what your after and know what you want done, I use this local garage just near cricklewood. They are really cheap, i usually pay about 25-35 labour for a service including fuel filter, oil change, engine flush, fuel filter air filter cabin filter spark plugs and anything else that you could class as a service, also brake pads are usually about £20 for all 4. Obviously you provide parts but I think the labour is really good. Also the last time I checked they wanted £50 for a gearbox oil change. Also when I had my old polo a few months back it was only £80 for a brake servo change when everyone else wanted £200 and they did a fab job.
If you know what you want they are a good place and never let me down yet.