I used to have a Birkin Seven which I fitted with a Lotus Twin Cam Engine on a Ford 2000E gearbox and towed it many miles on an A Frame going to track events/race circuits (this was in another lifetime in S.Africa)
When I made the A frame I checked that the gearbox would receive enough lubrication via the main shaft turning in neutral while being towed, it did PROVIDING the gearbox was full to its correct level (better to be even a little higher)
I never had a problem and on a long trip I would occasionally stop and feel the gearbox to see if was getting hot.....it never did, you can of course stop periodically and start the car so the 1st motion shaft flings a bit of oil around but as I said I had no problems.
I did have some nasty experiences with A Framing however and you must adopt a defensive style of driving, allow longer stopping distance, no sudden swerves and changes in direction and NEVER TRY TO REVERSE WITH AN A-FRAME HOOKED UP....you will only get grief.
I had a bracket fixed to the chassis to which the A-Frame attached, when not being A-FRAMED spot lights were fitted to the bracket for normal road use.
I do not know the laws covering A Framing in the UK, are you allowed on motorways?