Hmm! that is a tricky point, because it isn’t the value of the car that determines the quality of tyres that you should use on a car. I would suggest that you chaps didn’t buy your XJ40 just because it was cheap. you bought it because it is great car. The fact it is currently undervalued, makes buying the car easy to afford. However running costs are not so easy, and i guess tyres are a big part of that.
Jaguar have always made brilliant cars that handle remarkably well considering how heavy they are. and they are fast. and therefore they need to be made of good components. particularly tyres
I do sympathize. I had a series 3, XJ12 saloon which i loved dearly. I did over 140 mph with that monster, and it felt like it hadn’t finished accelerating. However i paid £ 2,500 for it and a new set of tyres was about £ 1,500 at the time.
Incidentally theyy are available in the sates from https://www.lucasclassictires.com/225-65WR15-PIRELLI-CINTURATO-P5-2256515L.htm