"294. Keeping control of a vehicle being towed
(1) The driver of a motor vehicle must not tow another motor vehicle unless—
(a) either—
(i) the driver can control the movement of the towed vehicle; or
(ii) the brakes and steering of the towed vehicle are in working
order and a person who is licensed to drive the towed vehicle is
sitting in the driver’s seat of the towed vehicle, and is in control
of its brakes and steering; and
(b) it is safe to tow the towed vehicle."
There's no problem controling the movement of the towed vehicle which is the case when an A-bar is used; as long as the steering geometry is OK the towed vehicle self-steers like a dream.
and (b) Who could ever be unkind enough to call a CMP unsafe?