Don't really keep up to date on laptops, so I don't really know which are the best options.
That said, I'd probably be looking at this with your requirements;
or if you can spend the extra 100 quid, this has a better GPU with RTX support;
The Ryzen 7 4800H in the Asus TUF is the better CPU though, so the extra £100 for the MSI GL65 is only getting you a slightly faster GPU with RTX support. If RTX doesn't interest you at all, then I'd stick with the Asus TUF.
Both only come with a 512GB SSD but I'd just buy an external HDD anyway, in whichever capacity you'll need. You can always swap out the SSD for a larger one in the future if you need/want, and have some extra cash. You can quite easily find good 1TB NVMe SSD's in the £100 range.
Curry's and Overclockers both offer finance. I'm not sure what type of finance either of them offers though, as I've never used them personally.
From a quick look, it seems like Curry's don't have an interest free option, and you might be paying 24.9% APR from Curry's unless they offer more finance options that I'm not seeing.
Overclockers seem to have an interest free option that you might be able to get with that MSI GL65 from them, but if not then looks like 14.9-19.9% APR.
Klarna or Clearpay are going to be your best options for finance IMHO, so you honestly might be better off looking for somewhere that sells laptops and uses either Klarna or Clearpay for financing, then hopefully finding a suitable laptop from there.