Towing a Patrol using a Patrol

[the evil twin]

Does that mean the trailers also legally suppose to have a system to automatically apply brakes if the trailer breaks away?

Yep... wot Firm said X2 (I included your post as a quote 'cause it was on the prev page)

All Vans, Trailers etc etc over 2,000 Kg gross will have a dedicated onboard battery for eleccy brakes or an onboard resovoir for air or an eleccy hyd pump if hydraulic and appropriate controllers so if the trailer disconnects the brakes are applied.