all of the following are available at most Tesco/Sainsbury's/Asda/Morrisons supermarkets:
You can get
Oatmeal biscuits not quite
Biscuits AB but you can do the same stuff with them, have them plain, with jam/pate on, and you can crush them, add water and they turn into a sort of porridge.
also you can get
Garibaldi "squashed fly"
biscuits (
Biscuits Fruit filled).
also
bacon grill is also available.
you can get some "very small" frey bentos
steak and kidney puds.
you can get small jars and tins of
pate'
you can get single servings of
Baked beans.
cup-a-soup, need i say more?
also
small tins of cake (not like the rations but similar).
packets of
porridge are now common as are
hot chocolate.
Dextrose tablets (near the medicines)
Apple flakes YES you can get them! ( in foil packets, "Fruitabu" (Kellogg's) Called "apple crisps" but it's just dried apple!
the tricky Items to copy/find are:
vegetable stock drink (though a stock cube like "knor"/"Oxo" would be similar).
Screech (orange/lemon drink powder) closest thing probably Lemsip
(better for you though).
Oatmeal block (though it's really just a very,very sweet hobnob!)
basically
for cheap rations I'd just go shopping and see what you can find!