Are you sure that you've exhausted all the possibilities with sandwiches? Have you, for example, tried stilton and mango chutney in a sarni? (Delicious!). Or cream cheese, crunchy peanut butter and celery?
There are some recipe suggestions here
https://www.bbcgoodfood.com/recipes/collection/lunchbox-recipes
but if you'd prefer something you can simply buy in a supermarket, try the wraps (instead of sarnis) from the sandwich counter or check out the hot chicken portions (etc) from the rotisserie counter. (Several supermarkets seem to be doing stuff specifically designed for a hot 'lunch on the go' these days).
If there's a Greggs near to where you work, try their hot pizza slices or their savoury slices.
If there's a microwave oven available for staff use where you work, look out for things like Jamaican patties in corner shops (where they're often easier to find than in the big supermarkets).
Back in Asda, Tesco, et al, take a look at the cooked meat counter to see if they sell slices of ham & egg pie (etc). Also look for the variations on standard pork pies that you sometimes come across in supermarkets (such as the ones that have got cheese and pickle in them).