Fighting back against supermarkets

Our local asda and tesco, have the same price on a warburton's toastie loaf or £1.35 (As of Jan 2012) - they order these in by the hundreds/thousands every week.
Our local petrol forecourt shop have them at £1.20 - they are consistantly lower in price than asda and tesco - they order in just 30 loaves every week.

If asda and tesco are "saving you money everyday" and stating "every little helps" - why are their prices for a warburtons toastie loaf artificially higher than local independants that buy from a nearby cash & carry (where the current trade price is 92p) and in considerably lower volumes each week?