If you on a rope or bar (breakdown/recovery) it must be taxed/mot'ed/insured and roadworthy as vehicle will have a driver inside whilst being towed

If it on a trailer (all 4 wheels off ground) then nothing needed except the towing car must be insured etc etc

If using a chariot or A frame then towed vehicle is classed as a trailer and only requires to be safe and have legal tyres on any tyres in contact with the road

(Methinks that is correct)