If you're towing it using an A frame, AFAICT it's a trailer. However, it must meet the requirements for trailers under the construction and use regs and so all the brakes must work while under tow (which normally needs some extra cabling). See
http://www.towtal.co.uk/aframe.asp for some info. Even if it is a trailer, it still won't be completely legal because the brakes must also be "auto reverse", i.e. automatically disengage when the trailer is reversed as do the brakes on a caravan.
If you're towing it on a rope or a rigid bar, it's a car and AFAICT it must be insured but need not be MOT'd if you're taking it to a pre-booked place of repair. However, I'm not sure about the tax situation. Pragmatically, you can't tax it without an MOT and you can't get an MOT until it's repaired. I know that you can drive an untaxed car to and from a pre-arranged MOT test, but I'm not sure about whether that also applies for places of repair.
Of course, the car must be safe to tow!
HTH,