I also got around 10% off here in belgium. Went to 2 different dealers and both of them offered nearly 6% straight away, at the first intend to buying a new car. When I had made my mind up I just pushed some more to get to 10% off.
Maybe they offer less discount in the UK as the base price is allready very low. Just compare your prices on base models to Belgium prices. I know that for an M135i its about €5000 difference..