Regulation 103 of the Road Vehicles (Construction and Use) Regulations 1986: "No person in charge of a motor vehicle or trailer shall cause or permit the vehicle to stand on a road so as to cause any unnecessary obstruction of the road".
If nobody complains (and the council's 'Mr Jobsworth' doesn't walk down your street), the question of whether the trailer is causing an 'unnecessary obstruction' won't arise. If it does, it's likely that a court would rule that your trailer is parked in contravention of Regulation 103.
However, all 'vehicles' (including caravans, trailers and skips) parked on the road at night must display lights. The exemption for cars, motor cycles and goods vehicles (facing in the direction of traffic on a road where the speed limit is 30mph or less) does not apply to other 'vehicles'. So, leaving your trailer on the road at night (without lights), even if it's not an 'unnecessary obstruction', contravenes Regulation 24 of the Road Vehicle Lighting Regulations.
Chris