When I left England in the Seventies they were just going over to A4 as a paper size. I don't know if they ever completed the transition. Before that there were two main sizes 'Quarto' (strictly Crown Quarto) which was 8 x 10 and 'Foolscap' (strictly Foolscap Quarto) which was 8 x 13.
There were dozens of paper sizes. We used 'Double Elephant' a lot for drawing. The largest was 'Antiquarian' which was huge (about 6ft x 4ft). We made an Antiquarian paper dart once and launched it down Gower Street from our 3rd floor office window.