Nope you can tow whatever the manufacturer puts on the plate as the maximum towing capacity and for 90/110's it's 3500kg with over run brakes and 4000kg for coupled brakes and it's the same towing capacity for both of them. There is no difference.
The 85% rule would bugger a few Artic tractor units as well.
Over run brakes work on the principle that when you brake the momentum of the trailer makes it run on and pushes a lever which applies the trailers brakes. Close Coupled brakes are usually air brakes such as are found on articulated trucks where you plug the brakes into the towing vehicle and all the brakes vehicle and trailer are on the same braking circuits.
class 1 hgv involves towing, class 2 dont. a lad at work was put through his class 2 but was not entitled to drive a transit towing a coring rig. strange eh?