I believe that as Nose weight is measured on the caravan's hitch. I don't think Plod or VOSA have any equipment capable of measuring the nose weight when using a Bike rack hence you don't get pulled.

I had a towbar on my 1.0 litre nova, pulled quite the thing

I fitted a rack and the vertical loading weight was 50kg, I never had a problem but i did put a strap from under the footpeg through the top of the back door and secured it to the floorwell to stop the "flex" that you get with the rack. I would think after continued use the towbar/rack would suffer from metal fatique and you defo dont want your pride and joy passing you on the road!

Nose weight and towing weight are to completely separate things.

Nose weight is the amount load you can pull down on the ball (as already mentioned) which is dictated by how you load your trailer (in front or behind the axle) In the case of specs the limit is very relevant as the total weight (plus any leverage from the frame pushing the bike away from the back of the car) is acting on the ball / tow-bar. The limit is set by the mounting points of the tow bar and the physical strength of the ball. Most have a max of 75kg, but i have never herd of one snapping despite lots of people regularly exceeding this by a massive amount. As far as Mr plod goes as long as you don't exceed your laden weight for the car then i don't think you are going to get nicked. (and as mentioned they have no way to check it anyway)

Towing weight is the amount of weight your car and trailer combination may weigh in total (including passengers and load in the car as well as trailer) so your maximum trailer weight is: Gross towing weight, minus total laden car weight-including passengers (whatever is left is the maximum your loaded trailer can weigh) The 85% thing is a caravan club guideline and on some cars can leave you heavily over weight despite being within it. If Mr Plod stops you, and escorts you to a weighbridge and finds you over your Gross towing weight the fines are huge!

Cars that are not EC approved for towing normally means that the manufacturer hasn't put in mounting points. The law at the moment means that aftermarket tow-bar makers must use the manufacturers mounting points, so if the car manufacturer doesnt make one, then nor can anyone else.