There is *no* "legal weight limit" for cars in the UK. See:-

Just because there isn't a specific infringement, doesn't make it legal. I
bet if you towed a full 3500kg behind a car they'd still do you. Whether it
is under construction and use, or just simply dangerous driving or whatever,
they'd still issue a prohibition and may prosecute.

Saying to the magistrate that the fact that you've towed a 3500kg trailer
whilst the vehicle manufacturer says anything over 1300kg is dangerous, and
you'll see what I mean.

Christian.