I regularly shop in both Lidl and Tesco and have done so since Tesco rearranged their store a few years back. Some items I prefer from one, others I prefer from the other and some items I'll buy in either. There's no huge savings in my experience. The so-called staples (bread, milk, butter, etc) are exactly the same price in both. In some cases Lidl might charge a little more for slightly better quality (e.g. the 4 ply shit roll) and in other cases they're significantly cheaper for similar quality (e.g. laundry liquid / fabric conditioner) but there are other cases where Lidl only stock poor quality options (e.g. shower gel, most toileteries). Lidl also have a habit of running out of stuff for lengthy periods... although, since Brexit, that also happens in Tesco. Of course, the main difference between the two is that Tesco generally offer a choice of brands whereas Lidl make the choice for you.... although some choice has crept onto their shelves over the years e.g you can now have Hellmans or Batts mayo, Lyons or their generic tea bags, Brennans or their generic sliced pan, etc...
Fresh fruit and veg is shite in both despite both significantly investing in partnerships with local importers / producers. Your local greengrocer / foreign shop / farmer's market/shop invariably offers significantly better quality at slightly higher prices, in my experience.
