I have much hands on sampling exoerience of a vast variety of wine, only through pure enthusiasm.
Aldi/lidl very limited range, not very complex flavours, no length (residual flavour).
Sains good selections but at the lower price range not so much. Either too sweet of too high in alcohol. 14% is almost port strength. You will really siffer the following day.
Waitrose own brand eg ordinary claret quite good.
M&S, if they have 20/25% off on six bottles or more you can also combine with reductions, eg, Petit Chablis reduced £12 to £8, 25% for 6, got it for £6.
My favourite is the Co Op, the price point, free trade and varietal range is deceptively good. Their fairtrade wines, their chardonnay, viognier/ chenin blend, malbec, carmenere (lost grape of bordeaux) and pinot rose, all stunning and under £6.
Obvs, you will have to buy one of each and pick you favourite. Tough job, good luck. 