I have put together a shop of 21 branded commonly brought products and compared the price at the 4 largest supermarkets, Tesco, Asda, Morrisons and Sainsbury's. Here is the table:
Asda the cheapest, next Tesco, then Morrisons and Sainsbury's the most expensive. These products where chosen to be directly comparable between stores so there is no excuse for price variation.
Meanwhile Aldi and Lidl stock fewer of these items, but I did manage to check out some of the prices they had, see table for Lidi prices:
As you can see, on branded products Lidl is more expensive than Asda, Tesco and Morrisons on the same basket of goods, of course it is their own brands where they can really show a saving. Interestingly on this basket Tesco's is slightly cheaper than Asda - which shows there is very little price difference on branded products between the two.
Aldi meanwhile had even fewer branded items, of the entire list they only had Carlsberg, Marmite, marolade, McVitie's and the Semi skimmed milk, on this basket they were £6.39, still 10p more than Asda on the same basket.
Branded products sold nationwide are usually chosen to compare prices as they are directly comparable across a range of stores, however I will attempt compare own-branded products.
Ps Here is a comparison of the how busy the Aldi store was compared with a Tesco express next door - tells the story of UK retailing at the moment.
Here is the Aldi - not rammed - but plenty of people.
and here is Tesco (same time, same place)...
Comp shop 04/10/14