OK, well lunch from a high street place won't cost you're than £5/7 including a drink. Even going to a restaurant/cafe for a lunch and soft drink won't cost you £15.
For stuff like that, I would do two things. Firstly, go on to tescos/ocado/sainsburys websites and put together a trolley of your usual sort of weekly shop. Obviously you won't know the best/cheapest brands but it will give you an idea of that basic cost.
There will be small versions of supermarkets within a few mins walk of anywhere central so you can also buy lunches there.
Then look up menus on Pret a Manger, Eat, Wagamama, Starbucks, M&S, Costa, Leon to give you an idea of the sort of lunch food and prices that are on most street corners.
Go to the council websites for any of the areas you are looking to rent/buy in. They will give you the council tax bands.
Go on to rightmove.co.uk and look up places to buy/rent in those areas. The website then gives you a very basic average costs for utilities etc.
At least that way you can have a better idea of a starting budget and expectations.