That's assuming the trailer is attached to a vehicle.
An unhooked trailer is not a vehicle, and nobody can obstruct the highway. For example, a skip can only be placed on the highway with a licence from the council.
It is s.137 of the Highways Act 1980 that states it is an offence to obstruct the highway.
I found this, which may help:
Can my neighbour park a caravan/camper van/trailer in their garden and block my vision/light?
A
A person can lawfully keep a caravan/trailer/camper van on their property without the requirement for authorisation but should the parked vehicle stick out into the highway by any amount then it is considered that the vehicle is obstructing the highway. This is then an offence contrary to the Highways Act 1980 and such is enforced by the Local Authority's Highway Enforcement team.
http://www.nelincs.gov.uk/AtoZ/atozDetail?ID=7 652#9528