There are shops in cornwall!
Could you not arrange for tesco to do a food delivery just after you arrive? We're doing this, so when we get there within a few hours milk and so on (including o oil, ketchup. loo roll and pepper) will be delivered.
Also, on the way down, on the motorway in particular, there are service stations with M&S simply food.
Whereabouts in Cornwall are you going?
There's a tesco in padstow and wadebridge. Other supermarkets in bodmon and Newquay, and generally good mini markets in smaller towns and villages.
I will be taking the bare minimum. Will make sure have headache pills for me, medicines for the children (i.e. calpol and nurofen), nappy cream, wipes and nappies. A few books and toys. Clothes (including more autumnal wear in case the weather is crap - joggers and wellies).
All the rest can be bought when we get there.