We use Sainsburys for our cat - have done for a few years and seems good value. Touch wood, never had to claim and she only goes to the vet once a year for her jabs which is about £40.
Thing with Sainsburys is they jack the price up quite steeply each year. Last year when we looked around, there were loads of places cheaper so rang them up to see if they could beat it. Rather strangely, they couldn't if we renewed, but if we cancelled and took out a new policy it went back to what we had been paying and we got £20 of vouchers (which turned into £30 when they hadn't arrived after 6 months and I complained).
This year, the same thing happened - couldn't beat the price but would let us cancel and take out a new policy, no vouchers this time though