ATV were into 525 for the exports. It was ABC that was the main commercial colour experimenter, in cooperation with the ITA (late night transmissions from Croydon). They did it all but with the BBC well advanced with NTSC they concentrated initially on SECAM.

I knew someone would know. Thanks I met a lovely old chap a few years ago who worked on VHF NTSC Colour at the BBC. He remembered how good it was.