I don't know about the UK but here could probably get a decent aftermarket set for that kind of money. You could try calling BMW corporate directly and try your luck with a goodwill warranty but it may not work.
You could go down the path of repairing it too, if the crack isn't too bad. There's companies that go door-to-door here, that will professionally repair and repaint a rim depending on the extent and location of damage. A good option if you're looking to save $$$ (sorry, can't find the character for pounds).