@MayaLuci There's a significant improvement in the best available laptops when you go from £400 to £450. If you really want to stay under £400, I can try to find something suitable, but you'd be compromising in a couple of ways.
This is the cheapest good option I saw, for £430 if you can find it in a store near you. It should be able to handle all Sims 4 packs together on medium settings without any trouble. The hard drive is small overall but can certainly fit the entire game plus mods and custom content.
https://www.currys.co.uk/gbuk/computing/laptops/laptops/acer-aspire-3-a314-22-14-laptop-amd-ryzen-5-256-gb-ssd-black-10209211-pdt.html
An alternative with the same specs and likely almost exactly the same performance is this one, for £450:
https://ao.com/product/x512daej265t-asus-laptop-silver-80137-251.aspx
If you have more questions, please feel free to ask.