How many are you feeding, OP?
Yes to tinned fruit, rice pudding etc in for backup
I'd definitely have a look at the reduced section as often as possible.
My top tip would be instead of looking at it as £20 a week, work out the monthly amount and make a meal plan for the month. It helps to get 'ahead' of the reduced section stuff ie have a little stock in the freezer to use, and then buy more as you go, making the next meal plan off that stuff, if that makes sense - so you aren't going out looking for a specific item for your meal plan, you've made a meal plan off what you already have.
Top cheap dinners tips I have are
Home made pizza - super cheap and can use up whatever veg / scraps of meat are lurking. Grated mozzarella best but I have definitely used just cheddar or a mix before and nobody noticed. I use Delia's four season pizza base recipe. Could also use a wrap, a pitta bread etc as a pizza base.
Scotch pancakes - feel like an indulgence and handy in a pinch - I have been making these for years and after much tweaking this is the recipe I use.
360g self raising flour (or plain plus baking powder)
120g sugar (granulated fine)
3 eggs
1/2 pint of milk (any milk - I used some powdered milk from baking this morning, made up with water!)
Slow cooker wise you can't beat a whole chicken or a whole gammon joint (although I have never successfully made a chicken last more than a meal and a few sandwiches for our family of four good appetites, the stock that it produces in the slow cooker is good for risotto etc).
If anyone is in NI, and happens to live near a Lidl - the new Lidl app is a great way to stretch a small budget - you get a 'scratch and win' card in the app every time you shop and win a small item every time - so I tend to divide my shop up eg one day I will go for the milk and bread, another day for the eggs and meat etc - get something free each time so this week so far I have had a shower gel and some biscuits - every little helps when the budget is tight! I don't think it is the same on the mainland but maybe someone could let us know.