Buying one of OCUK's prebuilt PCs. Could I save money on the OS?

[Elzzar]

I currently have a pc with a 980ti which is dying, main reason for my upgrade. I'll probably to a 3070 or 3080, not sure yet, but I am wondering if I could use the SSD which has windows 10 in this PC when I get my new one and just transfer it over, or would that cause issues? Windows would cost an extra £110. I am thinking OCUK might want the OS so they can do tests etc. too? Not sure how it works