It all depends on your income. The average income of a pensioner household is currently around £29,500. This average is, of course, very misleading as we know there are several million pensioner households dependent on state pension and pension credit and their income is likely to be under £10,000, while there are many others, with two pensioners, both with good occupational pensions, whose pension income will be £50,000 or over. Where do you fit along this line?
One company that did research came to the conclusion that the average pensioner household spent 14% of their household income on food. But the better off pensioners probably spend less than this and poorer pensioners more.
What people buy every week also varies immensely, not just according to income but also all sorts of other factors. The importance people place on food. Two households on the same income may spend different amounts of money on food because one just wants to spend as little as possible on food and their food decisions are entirely price driven, another may have ethical or wellbeing concerns and be prepared to cut down expenditure on other things to ensure the food they buy meets their ethical and well being standards.
I do not think the figure you seek exists in any meaningful way. I doubt there is a shopping list that can be defined as representing the range of foods bought by a significant proportion of retired people, which is what I think you are looking for.