I am not saying the VAT concession doesn't exist, but I have never come across it myself. If it does still exist, it must be a well guarded secret
OTOH, if you do export something out of the UK, and take it out personally via a port of entry, and have the VAT receipt handy, you can get the VAT back from Customs on your way out. This is pretty standard around Europe, though the degree of difficulty varies greatly and in some cases you have to apply by mail and the refund arrives some months later. However, in this scenario it is obvious the goods are being exported, by the fact that you are physically on your way out of the country...