British (and all European) law requires that if a towbar is to be fitted:
- the car manufacturer must have approved the car for towing;
- the towbar must be fitted to the attachment points specified by the manufacturer;
- the towbar must be capable of carrying the full towing load specified by the car manufacturer.
A car manufacturer does not have to get this approval - it's something they do if they think there is a marketing benefit in having the car approved for towing. For small-volume vehicles or ones that don't seem likely to be used as tow vehicles, many manufacturers don't bother getting approval and so it is not legal in the UK to fit a towbar to them - even if it is not going to be used for towing.
As far as I know, the situation with 1st/2nd Gen Minis is as follows:
Approved for Towing
Cooper and One hatchbacks, R50/R56
All Clubmen , R55
All Countrymen, R60
Not Approved for Towing
All Convertibles, R52/R57
All Cooper S hatchbacks, R53/R56
Coupés, R58
Roadsters, R59
If anyone knows the position with 3rd Gen models, please add it.
I think the Cooper S hatchback models are not approved as the central exhaust prevents a typical European towbar being fitted. Mini obviously thought the Cooper S Clubman was the sot of leisure/workhorse that might be used for towing, so they went to all the bother of fitting twin exhausts to leave room for a towbar:
There is obviously a bit of 'injustice' at work, since many of the un-approved models have exactly the same rear bodyshell as the approved ones, so a towbar would probably bolt right on - but it wouldn't be legal.
One double check that can be done is to look at the VIN plate sticker in the left door shut. Here is the one from my Roadster:
This lists four weights in kg in the following order:
- Gross Vehicle Weight (the maximum loaded weight of the car)
- Gross Train Weight (the maximum loaded weight of car and trailer combined)
- Front Axle Weight (the maximum loaded weight of the two front wheels), often with '1-' in front
- Rear Axle Weight (you can guess this one, can't you?), often with '2-' in front
In this case, the second line is blank, because the car is not approved for towing so there isn't a Gross Train Weight. This is pretty explicit and, for example, is something that any VOSA roadside inspector (the people who check cars towing trailers/caravans) will know - in case anyone was thinking that they could buy the towbar for a hatchback and bolt it onto a Roadster.....
None of the above applies in America where they have a completely different, and much less cautious/strict, system of towbar rating/approval. So they can produce and sell 'hitches' (towbars) for all the models that are not approved for towing in Europe. And, no, an American hitch fitted to a car in Europe isn't legal because it's American!