Thank you all. I had checked Google Maps and Apple Maps prior to writing the post. My concerns were:
a) some of the stores appeared permanently closed but still came up in the search results (I did end up in a newspaper article about some disused Morrisons being used as a drug den now though!)
b) the maps generally did not offer vertical topographical features like gradient and stairwells at tube stations for example. I will have three to four suitcases to fill up on a range of goodies* which can be a little unwieldy to move around with. I have previously done similar shopping trips like this in other UK cities before catching a flight back to where I live outside of the UK but haven’t found a great option near Heathrow - and the gradients can sometimes be problematic (had a gradient issue near Gateshead few weeks back, for instance!).
c) others has asked similar questions in the past On FlyerTalk but the replies referenced supermarkets that have since closed - and so I thought it appropriate to try to seek some additional local knowledge, particularly given that the responses are likely more vetted than those on Google. I’m sorry that my thoughts were incorrect. Please forgive me and I’m sorry for the faux pais.

For full context, I haven’t decided whether I will be transiting through London, or spending a few days. My plans are unfixed at the moment. I haven’t organised a hotel but I might do (in which case I might organise a Ocado, etc).

Thank you for your help in giving me some guidance. Please forgive my asking and believe me when I tried looking at Google Maps beforehand.

I really appreciate the help you offered even though I seemed to have committed a faux pais!

*Irn Bru; Percy Pigs; Fondant Fancies; Cadbury Caramel; big tubs of Cadbury Roses. Essentially, lots of stuff from home to take back with me.

EDIT: I Might not do all 3-4 suitcases at once! I might make multiple trips!