You can mail order it from Castrol Classic Oils on 01954 231668.
Some people say that the Silkolene and Motul oils are better oils being proper fully synthetics but I'll let oilman comment on that.
Also my local BP garage is now selling Castrol Edge in 10w/60 specifically for BMW M-cars - I suspect you'll find this Edge oil in your local BP garage?

OILMAN - IS CASTROL EDGE 10W/60 JUST THE OLD CASTROL RS 10W/60 RE-BRANDED? OR A TOTALLY NEW OIL ( shouting as it is a long way to Cornwall .... )