Even flying out of the EU, it is hard to find many 'duty-free' drinks anymore. But, having costed them in Waitrose beforehand, I found 'duty-free' in Gatwick was the same price. 'Tax-free' was even higher. Just don't expect anything to be cheaper at the airports in London.
Now, coming back from abroad - that's a different story, much cheaper in many cases. It pays to know the price before you buy!!