The manufacturer should be able to give towing weights, they may advise that you follow the weights calculated for a commercial van.
As for towing a car with an A-frame, I looked into this some time ago and found The car being towed would need to be less than 750kg to be legal. If it is over this the A-frame can only be used to tow to the nearest safe place otherwise you have an over weight trailer without brakes.
Check the link below out and have a chat with the caravan club.
https://www.gov.uk/government/public...van-the-basics