I find this at my Tesco in B'ham but not sure is nationwide but my guess it should be.
Only 56p can't go wrong with this. Bought one for me and one for my wife's car.
Summit - Magnetax Car Tax Holder
Easy to install, this magnetic tax disc holder holds license securely in place and allows quick and easy tax disc changes.http://www.amazon.co.uk/Summit-Magnetic-Licence-SMAG-Magnetax/dp/B002OM4G84