I've found Iceland deliveries brilliant too. I used them a lot pre-lockdown and couldn't get a delivery slot for a while, but things seem to be almost back to normal now
I think they've added loads of delivery slots, taken on plenty of new people etc.
Is there any way of buying the non-branded stuff and using it in a way your DP won't notice @AlohaMolly?! Admittedly probably not with a can of Coke but with any of his other treats? Or just giving him a budget for his treats and saying buy what you want, but we have no more than x amount to spend, it's up to you whether you get 20 treats for that or 2. DP is awful though for scoffing leftovers or entire meals if not watched, I completely sympathise with that, it's very annoying when I've bought something to last us a few days and he shovels it in absentmindedly 
There are loads of great recipes to make your food go further on MSE here (the whole site is brilliant, but that's the link I had stored), here and I second @RhubarbTea's suggestion of Jack Monroe. I know she's not everyone's cup of tea on here but she has some amazing ideas for stretching your budget a bit further for tasty meals.
Poundland (in store or online) is good for branded treats, shampoo & conditioner, craft stuff for kids and gardening stuff... I don't mind so much if the DTs break a £1 raspberry bush as I would if it was more expensive!
I've bought my DTs massive bundles of clothes via Facebook as people were getting rid of loads, helps them not go through them as quickly if we can rotate a lot.
We use the library a lot, the app when it's closed or actual books for the DC when it finally reopens!
Has anyone tried Approved Food? I haven't given it a go yet but have heard good things about it...