As I said before there is no legal maximum limit. The law does not state that if you exceed a specified trailer weight it then becomes automatically illegal. I think you'll find in fact that there is no maximum trailer weight specified (as a recommended maximum) for many cars, especially older ones.
I quite agree (and so does the URL I posted) that if you exceed the manufacturer's recommended weight it will be used as part of the evidence if you get prosecuted for having a dangerous load or some such. however you can't be prosecuted specifically for 'too much weight' because there isn't a weight limit.