Have you tried inputting your part numbers into http://www.cooper-bmw-parts.co.uk/ to see what they're selling them at (just to compare of course).
Some parts will be available from ECP/GSF, but sometimes they're cheaper, sometimes they're not.
You can always negotiate with the parts department yourself - or ring around various dealers, as I know the 3 closest to me never have the same price for any particular part.