You are talking about a car from another century, there are very few of those cars still running today.
The XJ6 was built back in the early 1970s when electronic fuel injection was a new-fangled gadget that few cars were fitted with, the early models suffered from electrical problems caused by very basic things like humidity causing relays to stick. A minor electrical problem could easily prevent the car from starting. The cooling systems were designed to work in the moderate British climate but if you took them to a hotter country they struggled to keep up. The XJ6 was difficult to work on, replacing some major components required removing the engine or lifting the car off the chassis. All cars from that era would be considered drastically unreliable by todays standards. Jaguar was a car that looked fantastic and got its owner a lot of admiring glances but after 5 years many of them spent more time in the garage than on the road. Yes the inline-six lasts a very long time, that wont help much if other stuff fails. The XJ6 was horrendously expensive to buy, the equivalent in todays money is about $350K, the only other car in its class was Mercedes which had a very different more conservative boxy look that appealed to a different kind of buyer.