Curiously, although descried as illegal in the earlier posts, there are still companies listing them for sale online, and suggesting that they can still be used, eg
here.
The confusion seems to relate to whether a wheeled towing 'dolly' is being used (such as those used by the AA or RAC to recover cars). In that case, the dolly is seen as a trailer and the car as a second towed item. Using two 'trailers' is not permitted except in the case of items less than 750kg, (a common-place method with farm machinery for example.)
With an A-frame bolted to the towed car, and where the only wheels involved are those of the towed car,
this discussion and also what purports to be a fact sheet from the Dept for Transport,
here, might suggest that using such a frame
is still legal, so long as brakes are operable on the towed vehicle. The last line reads "From the above I hope it is clear that we believe the use of "A" frames to tow cars behind other vehicles is legal provided the braking and lighting requirements are met."
But don't take my word for it...