May be a bit late now for OP's trip, but for anyone who's thinking of buying from Duty Free at Changi on arrival from LHR - don't bother.
It's cheaper at World Duty Free at T5 (yes, even though technically there's no more duty free in UK). Particularly so at certain times of the year when WDF offers extra 10% discount for "reserve and collect" (your item is merely kept aside at the customer service counter, you'd pay only upon collection).
Also prices at Changi Duty Free are higher for arriving passengers than departures.