G-ROAR;
I flew the Cabair banner system when it was first introduced. The CAA stipulated a twin and we jumped through months worth of hoops including trial flights and a lot of crew training, both pilots, observers and ground crew.
AFAIK the Cabair system is the only one approved by the CAA and they stipulated professional licenses with a minimum number of hours of underslung work.
Banner towing demands a lot of respect from those involved as it is incredibly easy to get it terribly wrong, with highly public, and probably fatal results.
PM me if you want to know more.
VH