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It's January, I want to save some money and so I need inspiration for my lunches.
I can't bring lunch with me as I run in most days and don't really carry anything.
What I need is ideas as to what I can buy from a supermarket that is tasty, reasonably filling and reasonably cheap. We have a microwave but it's used by about 100 other people so I rather not use it every day.
What ideas you got?

https://www.merchant-gourmet.com/products/range/ready-to-eat-pulses-grains/
These, or similar. Not that cheap but can be eaten hot or cold.
For maximum cheapness I'd imagine on Monday buying a load of bread, a pack of spread and family pack of ham slices would see out a week (assuming you have access to a fridge).
Depends on your food boredom threshold. My current lunches (for a few years now) are a tupperware pot of raw veg (always carrot, cauli, tomato and mushroom, plus other stuff like radish peas, edamame, baby corn depending on what's in the fridge), plus a bag of crisp type things. Then a banana mid afternoon. But you would have to prepare that and bring it in.
when younger, and feeling poor, I would buy a small loaf of bread and a pot of coleslaw, and just make coleslaw sandwiches, replacing bread and coleslaw as required. You do have to trust that your coleslaw and bread won't be nicked by co-workers (although bread survives quite well in an office drawer for a couple of days, I wouldn't leave the filling out of the fridge).
If you need variety, then it might be more difficult.
cup a soup and a loaf of bread of your choice?
Ginster's Cornish Pasty available in most supermarkets for just one of your Eengleesh pounds.
find somewhere that does jacket potatoes. It's unlikely to be as cheap as our canteen which is around £1.20 for a rugby ball sized spud with beans. Healthy, filling, cheap - tick
loaf
pot of hummous or prepack salad if any availible in the Oops section.
yogurt .
thats my supermarket food of choice for lunches
Greggs?
I can’t bring lunch with me as I run in most days and don’t really carry anything.
Get a decent running backpack then. And carry food in - there are lots of meals that don't need reheating.
get yourself to lidl (if you have one nearby)...
Pack of tortilla wraps ~80p
Tub of humus ~ 60p
Salad ~ 60p
Pack of Gosh falafel type things ~ £1.50
Squish it all in a wrap, roll it up and bobs your uncle. about 80p per wrap