Thought I would just update this with the outcome.
BMW UK paint guru said it was to "factory production standard" (knew he was going to come out with that bullsh*t). Again my argument was BMW are supposed to be a premium/prestige brand (confirmed by the sales manager when I asked), so how come the paint finish is worse than on the less than half price Toyota Aygo. Response was that BMW don't comment on the finish from other manufacturers. I mean WTF, BMW apparently don't benchmark against any competition!
Anyway, more to-ing and fro-ing and in the end I get my deposit back and we walk away from the car. Doesn't look like I'l be buying anymore new BMWs given the unwillingness to rectify quite a major problem. Again, it may be done to their powder process, or any other reason, but the fact is it is a diabolical finish that people should not accept. But the masses do and that's why BMW have clearly let their standards slip.