Conrac monitors certainly got used in the UK. I don't recall ever seeing RCA or Zenith here.
I've also seen Electrohome monnitors (Canadian?) in the UK. Not sure if they did Grade 1 monitors.
Melford was another UK manufacturer. Possibly a little later, 80s rather than 70s. I have pile of their 12" monochrome monitors here which I'm overhauling for the BECG's ABC-Thames project:
https://becg.org.uk/projects/abc-thames/ While we have some Melford colour monitors we'll probably use Barco for the colour monitors.
Valve (tube) colour monitors? I've seen a few from the 1960s made by Bush in the UK. Also the remains of a massive monitor in a metal cabinet that must have used a round CRT. In the UK and Europe we were comfortably into the era of rectangular CRTs when colour services started in 1967. A few roundies were used for experimental work in the 1950s and early 1960s. When you guys in the USA had a colour (color) service from 1954.
The oddest colour monitor I saw was an experimental one made at EMI, probably c1960. Three monochrome monitors, about 14", mounted in a huge cabinet with big dichroic mirrors to combine the RGB images. Presumed destroyed.