If I'm in early, I'll buy breakfast at work. A 7 item cooked breakfast is about 80p for staff and it's very good.
Some other food is sold at not much above cost, but you have to cook it yourself and I don't care to spend any of my lunch break cooking so I don't. I usually bring food in from home instead. The staff room has the basics - kettle, microwave.
Sometimes I'll get something from a nearby shop if I didn't bring something in. Tuna and rice, for example, is about £1.20 if you buy a pouch of rice and a tin of tuna from somewhere like B&M. Cheaper and healthier than buying a pasty from a bakery. Sometimes I'll buy a sandwich to eat while walking into work, but I don't like mayonnaise and it has become the fashion to put it in almost every sandwich. So sometimes maybe £2 - 2.50 for a sandwich. Sure, I could get a sandwich for £1 but I'd rather not eat tasteless cheese or reformed meat products in damp, cold bread.
The most annoying expense is when I've gone too long without eating something and need something quick that I can sneakily shove down my throat while working in order to avoid becoming Mr Grumpy. That comes down to a bar of chocolate, which is 85p at work. Just metres away, the same chocolate will be about 20p in B&M. That's annoying.
If you don't count the cost of food I bring in from home, I estimate I spend about £10 a week on food at work (or on the way to work).