I'd advise you to keep away from BMW dealerships both car and bike, they seem to be taking you for some kind of mug mate. Why not try an independent instead, so long as they are registered for VAT and use genuine BMW parts you are covered. If out of warranty, you can use whatever parts you like.
I replaced my rear disk on my previous 2004 GS and used a Galfer wavy disk which I thought was expensive at £79 inc vat (along with a set of Steptoes ferodo pads instead of the ridiculously expensive BMW Brembo items) .