round these parts, thin cement board is the norm for developer soffits.
https://www.roofingsuperstore.co.uk/product/non-asbestos-undercloaking-strips-1200-x150-pack-of-10.html
https://www.samuelkirk.co.uk/non-asbestos-soffit-strip-2400-x-200-x-6mm-15271-p.asp
https://www.resistant.co.uk/products/multi-pro/