Buy your own oil and take it with you!
BMW charge £120 for 6.5 litres of oil, whereas you can buy the exact same stuff for ~£52! Secondly, why would you want your air con regassed? You car must be a maximum of 3 years old? Mine is 6 years old and is still cold as hell! If you drop the air con regas and provide your own oil, the price should drop to around £380-£400. Then negotiate a little more and it should be yours for around £340.
Also, find a dealer that matches local BMW Specialists and get a quote from them, then take it to the price matching dealer and see what they will do.