Nice history
Thanks for the info, interesting reading, but I'l take issue with two small things:
System A not only had 65 per cent of the vertical resolution of later standards, but it couldn’t do colour.
System A did colour just fine, the BBC experimented with 405-line NTSC before finally settling on 625-line PAL and System I. They only broadcast System A colour experimentally, but it was more a case of "didn't" than "couldn't".
So did transmitting colour data for effectively half the number of lines, though that was the result of PALs alternation of the colour signal’s phase to help correct phase errors by cancelling them out.
Not quite, that is how SECAM works, it alternates the Y-R and Y-B line by line at transmission, transmitting at helf-resolution.
PAL transmits both on each line, simultaneously modulated onto the colour subcarrier, with full vertical resolution, just as NTSC does. Because the resulting phase modulation of the carrier is subject to distortion over the signal path, which in NTSC gives the classic red grass/green faces problem, PAL reverses the phase on each line. If there is, say, 10% distortion it ends up in the receiver as +10% on one line, -10% on the other, and up to about 20% that cancels out visually in the viewer's brain.
To get better cancellation, later PAL receivers used a delay line to save up one line's signal and electrically average it with the next one. That is what gives the half-resolution colour reduction, but it's done at the receiver, not in the transmitted signal. SInce the eye is much less sensitive to colour resolution than brightness resolution this goes unnoticed anyway.