Fish fingers and beans for dinner and pizza for tea, not the healthiest but it gives you until tomorrow to sort something. Would you consider improvising cloth nappies for day time use (terry style) and saving the disposables you have for night? Lots of supermarkets will also supply nappies in there change area, I've seen either packets left out, or a sign saying to ask if needed. Won't solve things, but will buy you a bit more time.
Keep trying the health visitor, she will be best placed to signpost to other services. There will almost certainly be a food bank covering your area, even if the nearest location has closed for CV19, something will have been put in place. There may also be more spercific parent/baby charities who can help with formula/ nappies.
As pp have said, check any point cards you have (clubcard, boots, m&s, anything will help get you through). And anywhere you might have left money/spare change, it all adds up.
Do you have any friends with babies? Even if they can't lend money they might be able lo share nappies or formula. Or your mum if you can all go round for tea? She might be will to help by giving food, even if she won't/ can't give money.
How old is your older child? Is their school age (even if not attending right now) ring the school and ask if they can provide emergency school lunch (packlunch to collect).