I've done some homework on the high street and have been more keen on some designs at chaumet and de beers. I wonder if you well travelled lot have seen any of their stores around Europe that may also offer a duty free discount. Shall I say I'm prepared to do a diamond tax run.
De Beers being that common high street shop.
As far as I know they don't have many stores in Europe outside London and Paris. Maybe Moscow and Kiev?
Perhaps DXB would be a better bet although presumably you still have to pay tax when you return to the UK. I don't know what tax set diamonds are liable for.
Otherwise maybe JNB?