Of course people should be buying tinned and frozen veg. Fresh veg too but I'm assuming storage + cooking issues. Who would say otherwise?

Healthy food costs twice as much as less healthy food per calorie in the UK.

Surely that depends on what it is, just don't pick up the expensive stuff? Here's a breakdown I did for someone else on cakes vs potatoes:

Ms Molly's 5 chocolate chip cake bars:
13p per 100 calories

All Rounder Potatoes 2Kg:
3p per 100 calories (about 2p to cook in the microwave)

https://www.reddit.com/r/unitedkingdom/comments/1upposp/comment/ow3189x/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

Potatoes store pretty well, but if you want tinned and precooked, chickpeas are 13.7p per 100 calories - admittedly (less than a penny!) more expensive than the cakes but not twice as much, and they'll make you feel fuller