All of the major supermarkets deploy the usual trick of marking up wines that have no business being sold for circa £8 to £10 a bottle then marking them down to create the illusion the customer is getting a bargain.
Articles from many wine writers, particularly Tim Atkin, suggest the supermarkets are leading us down a blind alley in terms of wine quality/price. Apparently there are a significant amout of wine producers who don't even bother with the UK market as they wouldn't be able to turn a reasonable profit due to the supermarket's pricing structure.
If you believe a fiver for a bottle of Wolf Blass represents a bargain then happy shopping, personally I'd rather up my budget by a few pound and go for something a little more interesting. Unless you have a time machine you're never going to get back the time spent drinking bland, mediocre, mass market wine.