As the coupling is generally bolted to the draw bar and the tow ball is generally bolted to the vehicle etc, I would have thought an appropriately engineered draw bar extension could be bolted to the trailer.
I think what Xk59D is saying is that swing away couplings dont comply with uk construction and use regulations. No matter how well engineered or manufactured if you use one on uk roads and there is an incident however unlikely or even not related to tge coupling you would still be in trouble for using a none compliant trailer.
For the ops purpose lopping a couple of feet off the front is perfectly feasible so no real need for a swing away coupling
Im not 100% sure but i think Xk59D could be right, there are some quite specific guidelines for uk trailer manufacture like a 3500kg hitch has to be a delta hitch rather than a single pole.
Im sure the information could be found if someone can be arsed to trawl the internet long enough
Tickler
