I have looked into this (and found Raffles to be a genuine guru on the subject). The biggest issue I have found is that Tesco impose such meagre credit limits that it is impossible to pay a large tax or VAT bill without breaking it into several components, paying them separately, and transferring cash across to the card regularly (ie every few days) to free up the credit.

Tesco have offered to look at the credit limit if I send them my accounts etc; frankly, life is too short, and I just can't be bothered. The fee charged by HMRC is whatever the opposite of icing on the cake is (the proverbial straw, I suppose).