It should be possible to make distinctive bakeries in towns and bakery factories, with the first only consuming Flour, with no production, while the latter produce packed food to be delivered to supermarkets.
Depending on the historical situation in Britain, you can set the start date for the supermarkets, and/or industrial bakeries to 1970 or so, and have normal bakeries up to the 80s they will disappear slowly then.
In continental Europe it is quite typical now that bakeries and Supermarkets in towns get frozen prefabricated products delivered, they just finish to bake. The chain would be: Flour -> Bakery: frozen goods -> Bakery store/Supermarket.
This is logistically easy, since the branches need only an automatic oven, customers get fresh products, stores have no problem with the short shelf live of bread, and can quickly.
A different remark, i don't think delivering to Supermarkets or small stores does fit very well into the abstract scope of the game. Delivering to Wholesales would much better however. This would be only a name change though. The tendency of simutrans to build the supermarkets beside towns, instead of into them, also would speak for this.
@skreyola,
thanks for the explanation. Doesn't the bread get stale immediately below 10 or 7 C, since starch crystallises? I'm living in Toronto now. It's surprising how different food in north america is from the rest of the world. I wondered how i got so fat in about 2 months, until i learned sweet things like cake or biscuits are really dangerous here, they replace normal sugar (saccharose) with fructose! Saccharose and Starch are filling up glycogen stores while the fructose increases lipogenesis.