With any used car I would always advise getting a proper independent check such as Car Vertical. 10% off, link at the top.
Book price for a V6 non -S convertible with 38k miles would be £21,670 so seems a little high but if it has options and is in good nick it might be higher. MOT history I presume you have looked at. Whether that is a measure of how the previous 4 owners have looked after it is up to you.
Service history should be every year or 16k miles which occurs first. Link here Https://osh.jaguar.com