You are FREQUENT visitor and have friends and family there that you cannot ask about basic expenses.
I am almost getting my bachelors degree. UNIVERSITY EDUCATED. Not that hard to look up cost of living.
Shall we all guess if you just eat First Class gourmet menu items or shop for bargains.
Groceries is just one expense. Transportation, health insurance and many other items need to be accounted for in your budget.
YES it matters if you are in a big city or a smaller place.
£40 - £50 for the basics.per week.or about £200 per month. If you have expensive tastes it can be a lot more. In particular eating meals away from home.
In summer it is usually cheaper to rent a room in a University dorm. Look around the area you plan to be for what is available . Most do not post this service online.
Doubtful Airbnb would be cheap for months and months. Only YOU know what arrangement you have for rent.
https://www.workgateways.com/working-in-the-uk/cost-of-living