Sainsbury's taste the difference are normally decent enough. I like the pinotage, it's normally approx £8, unless they're gearing up to make it an offer...
I'd avoid cheaper french wines, they always taste quite 'empty' to me.
One other option is to look into eastern European wine, not fashionable so cheaper. For example, Austrian wine is often pricey because it's niche and good quality. Most of the red is grown in the eastern edges of the country, which boarders on Hungary and Slovakia. So Hungarian wine is often similar without the price tag. Quite full bodied fruity wine. Can be tricky to get hold of in the supermarkets but worth looking online. (Slovakian also has a very good reputation on the continent, but trickier to buy here)
Also look at German wine. They have a quality system that must be written on the label: pradikatswein is the highest quality, with qualitatswein the second teir. We often find pradikatswein cheaper than qualitatswein, because the British taste for Reisling seems to be super syrupy sweet, so a drier version always seem to be priced down. I don't know much about which of their red wines to recommend though I'm afraid. I have to admit I've always been disappointed by Aldi / Lidl wines though, Morrisons around here seems to have the best German wine selection.
Oh and if you don't like syrupy sweet, avoid Jam Shed recommended above. It's possibly one of the worst wines I've ever had (sorry pp!)