Making it look like a deli and calling it a deli will make a difference.
In reality most supermarkets are the same. You can buy the same stuff in PFC, Tesco, Waitrose, Sainbury's, Istanbul, Costcutter, The Co-operative, Morrisons, M&S, Asda etc. You can buy cakes and bread in The Cake Store aka Slatters (what is it's real name?) just like you can at Tesco, Sainbury's and The Cooperative across the road but they have massive cues.
Most brands know that most people buy emotive purchases. Most small high street shops don't.
For an innovative retailer, just like the ones doing well in Brockley (which had 2) and Crystal Palace, I'm sure a well designed, well run, Friendly, sensibly priced deli would be an asset to the high street.
It works for Billings, despite the proximity of the supermarkets and another butcher in the high street.
Look at the comments after I posted the deli mock up on the forum a few years ago:
http://forum.sydenham.org.uk/viewtopic. ... 9&start=40
http://whatifsydenham.wordpress.com/200 ... -chemists/
http://whatifsydenham.wordpress.com/201 ... -sydenham/