Cheap and easy swaps for hard-to-find baking ingredients such as flour, eggs, milk & more

The past several weeks have seen grocery shortages in many shops most won't ever have seen, unless they're old enough to remember rationing like my dad (hi Dad!). With most of us at home, baking is a popular way to pass the time, but with many struggling to pick up their usual key ingredients such as flour and eggs, there's been renewed interest in wartime recipes, as well as vegan baking.