Well I'm 95% sure that the "who" is HRH Edward, Earl of Wessex. I'd guess the "where" is next to the Royal Yacht Britannia (but where it's moored is anyone's guess). The "what" is of course a facelift XJS so that's leads us to the "when" which I'd expect is the early '90s. As for the "How", a number of Royals used to drive an XJS it seems, including Princess Diana.
And if you've ever wanted to see Princess Diana with an XJS, you're in luck: Princess Diana and Jaguar XJS - Royal wheels
And if you've ever wanted to see Princess Diana with an XJS, you're in luck: Princess Diana and Jaguar XJS - Royal wheels