Far from being completely the whole story.....
as long as the trailer, the car on the trailer and the tow vehicle are less than 3500kg (combined)
AND the tow vehicle weighs more than the combined weight of the trailer and whats on it (the 5) you're all nice and legal... (providing the towing weight of the tow vehicle isn't exceeded)
Thing is a 5 weighs 820kg (wet) and a typical Brian James trailer weighs 450kg - which leaves you with a choice of any car/van/4x4 which weighs between 1300kg & 2000kg to be legal. So you buy a Transit, a Shogun (swb) or a Freelander, give the middle finger to the DVLA and Mr Brown, but unfortunately Moggy's Dad doesn't get your cash...
Checked this with the DVLA and the local old bill, and neither had a problem with it being dodgy... just a massive loophole in the rules
my money was better spent on bits for the 5 as well