I entirely agree with Ruddiger. Firstly I think theres some confusion around this thread - my understanding was that its not about the high street as a whole but the mini marts.
I too have attempted to offer some constructive criticism to some of the mini-mart owners. There seems to be a general feeling amongst some of the owners as to why anyone would shop in Sainsburys, when they sell a select number of items for cheaper!?!?! To me the answers obvious and its better that we inform and communicate (after all we are the customers).
As I mentioned in an earlier post - I would advise any mini-mart owner on the high street to take a 5 minute walk to the Turkish mini-mart on Forest Hill Road and take a look at how they are competing with the Co-Op a few doors down.