Hi Ruthib
I hope this could help you.
From what I have read there are two factors that restricts your towing weight
1; The vehicle manufactures specification on towing limits.
2; the vehicle manufacture specifies the maximum gross weight of vehicle..
I think this link to the government web site concerning towing weights might be of great help.
http://www.dft.gov.uk
To find out how much your car and caravan weighs, go to either one of these links for a more up to date list.
http://www.cuddles.abelgratis.net/kerbweights.htm or
http://www.whattowcar.com for a more up to date list.