I had reason to go to Penarth last night, and was amazed at the installations I found there. There is street after street after street lit by cast iron columns and what I think are Thorn Beta 4's. I must add however that all of them were running SON lamps, no mercurys were found
Those are AC Ford AC730 lanterns, likely to be mounted on AC Ford swan necks too. They are probably relatively modern and could have been SON from new. In Southampton these lanterns were still being installed as casual replacements until about 2004, always in SON, and were fairly popular casual replacements in the 1990s when the council wanted to keep cast iron columns top entry rather than cutting off or replacing brackets and fitting modern side entry lanterns.