I spent a lot of time looking into this subject when I owned a Merc CLS and wanted to fit a tow bar.
Mercedes are of the opinion that they don’t want people spoiling the aesthetic appeal of any car in their coupe range by using them as tow cars. Consequently, when applying for type approval, Merc does not include a tow bar amongst the items defined in the build standard.
(It’s an important but subtle point: the issue at stake is that of EU Type Approval, and not homolgation. The USA is not part of the EU so the rules on EU Type Approval do not apply there. I am unsure of whether the US has any similar rules about owners may fit to their cars.)
A large German company called Rameder sell tow bars, made to their specification by a Belgian company whose name I forget, but it isn’t germane to this issue.
Because the floor pan of the CLS is identical to that of the standard E Class saloon, Rameder went to the German Type Approval authority, armed with a sub set of the design data set for the E Class, together with a body of evidence that showed read across to the CLS model.
They were duly issued with Type Approval for their bar onto the CLS. Subsequently every bar that Rameder sells comes with a Certificate of Conformity that provides traceability to that Type Approval.
Note Mercedes was not, to the best of my knowledge, involved in the process, though they may have provided some stress calcs to Rameder.
On to the XK8:
Jaguar Cars Limited when they sought EU Type Approval did not include a tow bar as one of the items on the build standard list, and unlike the E Class floor pan, the XK8’s floor pan is unique and I would venture to suggest that it was not designed to take the stresses imparted by a tow bar, and certainly not one which relies on a single point mounting as that American thing appears to do.
If Jaguar had intended the car to be capable of being equipped with a tow bar, do you not think that they would have added one to the options or accessories list? After all they're jolly keen to extract money from owners for every other branded toy and option. The fact that they did not must tell you something.
As someone has already pointed out, the requirement for towing couplings to be EU Type Approved was brought in some time in 1997, and applied to all vehicles registered new in the UK after that date. I think the legislation also applied to tow hitches on trailers etc.
Thereafter it is an offence under Construction and Use Regulations to use a vehicle first registered after 1997 with a tow coupling (and that includes the ball) which is not EU Type Approved.
Any eagle eyed copper or indeed VOSA (or whatever it now calls itself) inspector, of which there are now many since VOSA had to become financially self supporting, will know that any XK, other than a very early one, should not be fitted with a tow bar, or indeed towing.
If you have an accident or there’s other cause for your car to be inspected then you will be in for a whole world of pain if you fit that tow bar. Your insurance will almost certainly be invalid, and you know how keen insurance companies are to avoid paying out.
On the subject of inspection you might find that a sharp MoT tester picks you up for having it fitted and issues an advisory.
The reason that early XK8s could be fitted with and use a tow bar legally was that their date of first registration fell before the 1997 cut off date.
Despite what you say there is no grey area: it is quite straightforward.
I doubt you’re interested but based on the German Rameder CofC VOSA was happy that I could add an additional ‘VIN Plate’ to my CLS which quoted its new max train and max towing weights – neither of which of course was on Merc’s original plate.
My suspicion is that a tow bar combined with the nose weight of the trailer on the XK8 will certainly put you in excess of the Max Gross Weight of the vehicle, and you don't even have a max train weight, and possibly in excess of the rear axle weight limit.
By all means fit that tow bar to your 2001 car but at least now you know all the laws you’ll be breaking unless you do like Rameder and go and get EU Type Approval for your imported tow bar.
N.B. This may all change after Brexit!
Final note of caution: if you use that electrical kit, then based on a quick glance at the installation instructions I think that you may have problems because Jag switches both high side and low side on some circuits. Suggest you proceed carefully on that.