Put it this way, I went back to UK recently and went to Tescos with my sister. My jaw dropped open and I nearly cried - it is so cheap compared to here! packet 6 sausage rolls - 1 pound cheese - way way cheaper, and nice cheeses too, brie etc yoghurt - way cheaper meat - a bit cheaper pre-packed salads - 1 or 2 pounds eg. cadbury chocolate fingers - 1 pound (five dollars in my corner store here) tinned stuff eg tomatoes, baked beans about the same Many items that are 5 dollars here - juice etc are 2 pounds there (about 3 dollars)
I don't know about eating out, but if you are going to the supermarket then I would budget about half to two thirds what you budget here.