This site discusses the BBC television coverage of the 1953 coronation. A single paragraph is devoted to the experimental closed circuit color broadcast. I agree that it was possibly a field sequential system. Here's the paragraph:
"As befits the coming generation, two hundred children saw the Coronation procession by the TV of the future - in colour. They were at the Great Ormand Street Hospital in London. By closed-circuit they received pictures from three TV colour cameras overlooking Parliament Square"
And the site:
APTS | 2 June 1953 - Coronation Day
Address:
http://www.apts.org.uk/coronation.htm
-Steve D.