Looking to sell my S40 and upgrade to an S90... a few questions?

[JackS40]

Hi! I currently own a 2008 Volvo S40, and love it to bits. It's starting to get a bit flakey though, as cars do with age. I'm looking at selling my current car and upgrading to a new S90. However I'm a little confused about how income tax works on selling cars - the S40 was passed down from my parents who upgraded to an XC60 a few years back. I've been looking at sites like this tax calculator (https://www.income-tax.co.uk/)but can't work it out myself, so could use some advice.

Thanks in advance!