You actually don't need much imo. In general, you'll need some folders - at least 1 arch lever folder for each subject at the start of the year. For bio and chem I have 2 arch folders each. One has notes and the other has revision. I think you'll only need 2 if you write a lot of notes in class and are given loads of handouts.
Get yourself file dividers and a small a4 folder for each subject, or you can instead have a 'day folder' (I don't know much about that but there's def yt videos)
For stationary I recommended buying a big pack of pens, especially in black as you're only allowed black in exams. I personally like 0.7mm pens so find yourself a good brand and buy in bulk. I also got myself some other stuff like post-it notes, highlighters (green and yellow), a ruler, correction tape. You'll also need a calculator for chem and bio. The calculator from gcse will be enough. I get most of my stuff from Wilko as it's cheap but stuff gets sold out quickly so go shopping in mid-August.
About revision guides and textbooks don't buy any until you actually start. My school gave us free textbooks and sold us revision guides at a discounted price.
Notebooks I use