Whist I am aware of others doing this, unless the Morgan brakes are connected, it's not legal. The towed vehicle becomes a trailer, and although the Morgan is light, I don't think the weight of it plus the A frame is under 750 kG (the point at which trailer brakes become compulsory).
Towing a trailer that should have brakes - but hasn't puts a great load on the brakes of the towing vehicle.
A frames used for recovery do, I think, have some exemptions but there are limits on the total distance they are used for.
