Yes, its cheddar and (shhh) a bit of red leicester or any other full fat mature hard cheese. Mustard and beer is usual but they never gave us the beer version at school.
Just try a test; only cost you a piece of toast. Normal cheese on toast and very thin spread of your favourite mild berry jam (not citrus marmalade). You'll be surprised. I was when I first saw it as an innocent schoolboy, newly arrived at a boarding school on the Borders....Warnng: nostalgia quotient high here