The last time I worked for a supermarket, it was the Co-op. I only ever had to deal with orders for ambient stock, but the stuff was either ordered from the national distribution centre in Coventry, or the regional centre (Cumbernauld, if I recall correctly), but the stock was shipped up to a more local distribution centre at Dalcross, just outside Inverness. From there it was loaded onto the local lorries for shipment out to the various stores in the region. Presumably the fresh/frozen stuff was similarly delivered to the "local" distribution centre to minimise the number of trucks being sent out to each store. It was some time ago, and I'm not certain that they use the same model these days, but rail would have been ideal, given the proximity of Dalcross to the main Inverness-Aberdeen line. Surely it's not beyond the realms of possibility to imagine that even smaller hubs could be utilised, with only the last few miles delivered by road?