It's a tough question, but as a starting point... if you're having a big breakfast in your hotel you'll not need a huge lunch, so £5-£8 will do it for a reasonable sandwich/wrap/salad/curry box + drink from one of the many decent chain outlets or a market. £3 if you're content with a Tesco/Sainsbury's sandwich meal deal.

Chocolate bars/crisps cost 60p-£1 in shops, £1.50 for fancy bars, £2-3 for proper bakery treats.

Dinner... well you can eat well for under £20 excluding booze at almost any pub, and at decent chains like Rosso Pomodoro, Byron, Honest Burger, Pho etc. At places like Pizza Express you can use voucher codes and eat for under a tenner. Local restaurants also often have good set menus for around £15.

Obviously you can spend any amount upwards of the above. I would say I typically spend £30-£40 on nice but not fancy dinners out with some booze.