Ready Brek was (and still is) a breakfast cereal that looked like sawdust, and you added hot milk to make a sort of smooth porrige. You could make it very thick and stodgy or as runny as soup. It was one of those things my Mum usually wouldn't buy, so of course I always wanted it. I think there was also a chocolate version.
The TV advert for Ready Brek had two kids going to school on a cold day, one kid was shivering and miserable, and the other one - the one who had Ready Brek for breakfast, of course - was outlined in red, and was warm and happy. I think the slogan was something like "Central Heating for Kids".