It depends on the method of towing. If you use an 'A-bar' the car behind becomes a trailer and doesn't need tax, MOT or insurance. The towing vehicle must be insured (including for towing) and you must have sufficient rights on your licence to tow a vehicle. If the car is towed by rope, then all the usual rules of the road apply - the towed vehicle must be taxed, MOT'd and insured, unless it is being towed to a pre-booked MOT test or to a garage to have the work necessary to pass the MOT.