At the 2 Expresses (not in petrol stations, just normal shops) I have used in the past around the area, I have noticed that they stock very different products. I kid you not - at one Tesco Express near a council estate they have a huge stock of cakes and multi pack crisps and you almost trip over the beer and other alcohol in the aisles where the shelves are already full whereas at another one in a more affluent area they stock chilled prawns and Tesco Finest belgian choc cookies (the ones that are about £2 a pack for 5 biscuits!) And hardly any crisps other than the giant packs of Kettle Chips type things. I did wonder if someone did some proper research or just made gross assumptions about the stock in different areas! So I would say that depending on the area the 'Express' is in will depend on the products they stock!
Incidently another Tesco Express I used to use on the way to the gym always had a much bigger selection of medicines ie ibuprofen, paracetamol, anti histimines, calamine etc etc and cereal bars than the other ones. Hmm.