I went on a wine course some time ago and the lecturer made a point that is obvious but easily missed: Aldi, M&S and all the rest do not make wines or spirits. Nor do they go to the effort of blending their own. So you can easily get a very good wine from Aldi or Lidl for example, which would be more expensive from M&S and even more from a specialist. It is also worth looking at Alsace in those discounters because the DOC rules require them to be in tall bottles which often don't fit the supermarket shelves - the region has been trying to change the bottle shape for that reason. So they often end up selling it more cheaply than a comparable wine in a more conventional bottle.