Co-Op currently have these instore for £3.99 per single bottle, which is as cheap as I have seen it. I drink the Sauvignon Blanc so can vouch for that. They also have the Cabernet Sauvignon & the Viognier for the same price. Cheapest I have seen these recently was £6.29 to £4.29 in Waitrose & they fly off the shelf at that price.
Loads of stock in my local store when I went in earlier. Doesn't appear to be online so no Quidco but can use your loyalty card instore.