Haven't done it lately but I think it's still feasible. Think you would need to make a separate visit to the HM Customs (near EK Check-in) in Terminal 3 prior to your flight as there is no way back to it once you're airside as far as I know.
Frankly, unless you are making a major purchase it's really not worth the hassle. All the VAT refunds are done through 3rd parties and by the time they've taken their cut you're left with about 50p!