You can legally park any trailer on a road. Insurance for any trailer is taken from the towing vehicle. Though there is no requirement for the trailer to be insured, I would still suggest doing so. The trailer must have reflectors, and should really be lit at night and parked the correct way. There is no requirement under the Road Traffic Act to insure a trailer as it only mentions motor vehicle. It may be prudent to do so separately depending on its value.

Reg 103 MV [Con and Use]Regs 86 however creates the offence - no person in charge of a motor vehicle OR Trailer shall cause or permit the vehicle to cause any unnecessary obstruction of the road. It has been held that a vehicle left for an unreasonable time may be an unreasonable obstruction.

A trailer so left as described has also to have at least one wheel prevented from turning by use of parking brake, chain, chock etc. Offence not to do so Reg 89.

Both of which, following a conviction, could lead to disqualification.

It does not need to be attached to another vehicle or registered. It does, however, need to be not causing an obstruction to other road users.