You should be able to put a set of 20's on there with 15mm offset and 5-475 bolt pattern if they are 20x8.5
Also you can get front 20x9 with 30mm offset and 5-475 bolt and rear 20x10 with 25mm offset and 5-475 bolt. Tires are gonna be more pricey than your 18's

Hope this helps.

 

Have you looked for factory jag wheels in the 20" dept? They're are just as equally priced, or are you trying to go for a real 'unique' look, such as the black powder coat?

I posted this thread awhile back, with some links for refurbed XK wheels all the way up to '07s.

https://www.jaguarforums.com/m_13815/tm.htm

 

I like them too, and I assume your XK is black also? I once had a dark blue camaro with black rims...so I can relate to the appeal.

I just would have to be real cautious about spending all that money on rims, when there is a chance that the bolt pattern may not be correct, or offset not just right for your taste, and it makes me queasy just thinking about it. lol

When I upgraded my tires on the 'ol jeep, I was pleasantly suprised when my tire dealer let me test mount the exact size I wanted, to make sure it would turn in the wheelwell w/o a rub, and it didn't stick out of the fenderwell. When that satisfied me, I ordered the brand of tires that I needed from him. Perhaps you can do the same thing. Ask your tire dealer if you can mount a similar rim and tire combo, then if you find that correct combination, you will order the tires from him, order your rims from ebay, or elsewhere, and he can mount and do all that other fine stuff for you. It may be worth his time and effort to do that for you. Just have to find the exact size and bolt pattern that you are looking for, and hope he has a wheel in stock, no matter how godawfulugly it may be.

good luck!