No. You can buy it. It is just that you should be declaring it when you return so that you pay the VAT........ but if you do not declare it... whilst you save the cash, it is technically tax evasion.
British citizen fiance got her Rolex in Heathrow and saved the VAT. We live in the U.S. so it is slightly different, but if she had just been on holiday, taking it back to the UK without declaring it would be tax evasion.