The Road Traffic Act 1988 allows Construction and Use regulations for pedal cycles only in respect of brakes and bells (section 81 (http://www.legislation.gov.uk/id/ukpga/1988/52/section/81)) and lights and reflectors (section 41(4)(b) (http://www.legislation.gov.uk/id/ukpga/1988/52/section/41/4/b)). Only the latter apply to cycle trailers. Thus, while it's illegal to tow a trailer more than 1.5m wide behind a motorcycle, there's no such restriction on trailers drawn by a pedal cycle.