I'm assuming that's 6% each not 18% per car! No dealer wants a cash deal these days but due to something called treating customers fairly they won't penalise cash or finance customers via big/small discounts. Let me know what your after and I might be able to help.
Considering you get 9% max front margin on a Cooper S the dealer sellingbthe car is makin a huge loss per car or has made a total are of it. Take the deal then tell me where it is and I'll go buy 3 too!!