You cannot order a car that’s not on the scheme, what you could do is buy your xc60 i guess, or leave the scheme and get an xc60 either new or 2nd hand but not on the scheme.
Personally I never keep my car more than 3 years and never add any extra’s as i just don’t see value there for me. After all most of the time you then paying the same for an older car than you are for a new one.
Which to me makes no sence at all. Unless the ap is high and you intend to buy at some point..