Co-op - the classic Audax pit stop. Quality has improved in recent years and even the smaller ones tend to have something vegan or gluten-free.
Greggs - bakery items great but often a long queue.
Waitrose - despite pricey baseline, the "yellow label" reductions are great value if you're there any time after 3pm. Toilets in most branches. Sadly no longer offer free hot drinks.
Wetherspoons - not everyone's cup of tea for various reasons but food arrives quickly, quality is consistent, vegan and GF options, water refills on bar. (This may change with Covid 19.)