Provided the driver has a full B licence - obtained before or after 1/1/97 they can tow a "larger" outfit on a motorway IF they display "L" plates and are accompanied by a driver with a B+E entitlement. The motorway restriction on learners doesn't apply to B licence holders practicing for B+E, C or D licences but a suitably qualified person must also be present - you'll often see HGV or bus learners on motorways.
By "larger" I mean an outfit where the sum of the two MAWs exceeds 3500kg or the trailer MAW (MTPLM) exceeds the towing vehicle's kerbweight - in other words the restriction imposed on post 1/1/97 drivers with B only entitlement.