Anyway today I:
1) dropped my prescription renewal off in the chemists and bought magnesium tablets with the points on my Boots card
2) went to buy balsamic vinegar, Barleycup and a packet of dried yeast in the delicatessen
3) popped into the Spar for 2 pints of milk
4) collected the local paper from the newsagents
5) bought some apples and a pomegranate from the small greengrocers.
Today was fairly flexible as Boots don't sell anything that I bought from anywhere else (but I did that first so I didn't forget) and the only items that needed considering were the milk (also sold by the delicatessen) and the local paper (also sold by Spar). Actually, thinking about it now, I broke my own rule by buying the apples on a sudden whim just before I headed for home but I didn't go to the back of the greengrocers where the fridge is and thus didn't go anywhere near the milk, so that was OK.
Other shopping expeditions can get more complicated...