Another alternative is to buy the duty free at LAX and not LHR -- for most things LAX is cheaper (but LHR has far bigger selection at the high end). Or an even better idea is to buy duty free upon arrival at AKL (you can even get 25% discount if pre-ordered on the web). AKL is famous for having some of the cheapest liquor prices of non-Asian duty free stores, so it is a bit silly to carry it with you from LHR or even LAX for things that AKL stocks (which would be all the mainstream brands).