just been through Heathrow today and in the Sony Center there they were also giving the VAT cash back prices but with one big advantage:
The price you pay is immediately the price excluding VAT, i.e. you don't have to claim the money back afterwards. They do deliver home for free, so don't worry about carrying around your Telly on the plane ;)
Of course not everbody gets the opportunity to shop there but I hope it is helpful to some. They also have netbooks, notebooks, cameras etc. there.