Saunders 05607 Recycled Hardboard Sketchboard - Brown, 23 in. x 26 in. Clipboard with Built-in Handle - Solid Drawing Board for Artists, Students, and Creatives https://www.amazon.com/dp/B002UXREKY/ref=cm_sw_r_cp_api_glt_fabc_PA665E3TZMPD798WVEBN?_encoding=UTF8&psc=1
That isn’t a UK link, but I’m sure you can find what you need locally. They are super common. It’s basically just a giant clipboard with a handle. Get it a bit bigger than your standard paper sheet size you will use most often.