From Thursday 28th March 2019, DVLA have stated that it will be a legal requirement for certain trailers travelling abroad to be registered.
When registered with DVLA, trailers must display a trailer registration number plate as well as the registration number plate of the towing vehicle.
Not every trailer in the UK will need to be registered. Registration will be mandatory for all commercial use trailers travelling internationally that weigh over 750kg in gross weight. Registration is also mandatory for non-commercial use trailers that travel internationally and weigh over 3500kg in gross weight.