I've moved many cars around over the last 2 years with my 'A' frame (I break Mondeos for pocket money) and when I looked into it, yes technically it is illegal for the reasons stated above, I even asked a police officer mate of a mate to look into it for me and he basically said they are only legal to use to recover a broken down car to a place of safety.
That said I've passed and been passed by a couple of traffic cars when using it and they haven't pulled me, I make sure its a 'legal' as possible by using a decent light board with the towcars registration on the back of the car being towed.
I tow using a Galaxy and my main advice is to take it steady, allow lots of room for braking, always make sure the steering lock of the car being towed is off! and dont try and reverse when towing with one as its a nightmare (you need someone in the car being towed to do some steering if you have to reverse).
As said above you are towing something that is of equal weight to the towcar so any sharp manouvres or going to fast round a roundabout and the car being towed will swing the towcar across the road... and off it....Just take it steady.

Steve