Could someone explain me the advantage of a non immigrant O-A visa to the non immigrant multi entry O visa which I currently have. Ilive here pretty much permanently, going back England for vacations about 3 times per year.

For you it might not have any big advantage.  But for someone who does not travel on a regular basis a yearly extension allows the person to remain in Thailand without need for any exit or border crossings or new visa outside Thailand.  On an extension it can be extended again within the country.  On the multi entry you have to exit every 90 days but you don't have to make address report as you would with the visa extension if you stay more than 90 days at a time.  When you stop traveling as much it might make more sense.