Quite a bit of what I'm saying comes from working in TV production -
although I'm on the sound side. But hear plenty from those who work
on the vision side of things. ;-) And it was certainly the case that
Grade 1 picture monitors continued with delta gun tubes long after
PIL was introduced domestically -- and stuck with the original NTSC
phosphors. As this was the standard the cameras were 'calibrated' to.

But there was no "original" NTSC red phosphor -- just a standard for it,
that the original red phosphors didn't meet.

The phosphors as used when colour TV arrived in the UK were known
as NTSC standard by the BBC. And were still specified for Grade 1
monitors for many years afterwards. Indeed, probably still are.

Forgive me, but how something is spec'd does not mean that the real-world
implementation -- regardless of its name -- meets the spec.

Of course, color TV was so late arriving in GB, it's likely that only the
improved red phosphors were used.