Not really, pa will show up fine on a black and white tv, as the chroma is lost in the line blanking. Some very poor b+w tvs with terrible filters might show a little high frequency noise but that’s it.
The big difference in the U.K. is that 405 lines was on vhf channels. The move to colour was also a move to pal, 625 and uhf.
625 lime receivers came out in the early 60s, and typically would also work with 525. Bbc 2 launched on 625 in black and white in 1964 I think, but didn’t add colour until 1967. 405 only sets stopped being sold by the end of the 60s.
Only bbc1 and itv transmitted 405, but continued until the 1980s. Most 625 sets were black and white 625 lines, and that slowly changed during the 70s.
A 625 black and white set from 1965 could pick up tv signals and display them fine (in b&w) until analog switch off around 2010 (phased across the country. The final 405 lines worked for 20 years.
Like ntsc colour was a sub carrier which exposed the colour difference signals. Unlike ntsc, pal switched the Phase on Alternate Lines, so reflections and other signal interference canceled out, like it does with balanced audio (and indeed balanced data in cat5 cables). NTSC didn’t have this correction so sets had a “tint” control to adjust the signal.
This led to the moniker “never twice the same color” for ntsc.
For a bit of trivia, David Attenborough has won an award for his work in each of the formats: B&W, color, HD, 3D, 4K.
He made snooker a (relatively) big sport by putting it on TV https://en.wikipedia.org/wiki/Pot_Black because he needed something which would advertise the benefits of colour TV. The matches needed literal-not-metaphorical colour commentary for those on B&W sets, leading to the famous gaffe "Steve is going for the pink ball - and for those of you who are watching in black and white, the pink is next to the green." http://news.bbc.co.uk/2/hi/entertainment/3629569.stm . The theme music was the "Black and White Rag" https://www.youtube.com/watch?v=4ri3utpSoRA .
Chroma is not lost in line blanking. It creates dots that crawl across the screen.
Yes the high frequency noise. Depends on how good the hf filter is.
Why would you have one? Want the image to look blurrier on purpose? Color TVs have one to block the color dots.
If you're deliberately displaying the image in black and white, the colour pattern is interference that should be suppressed.
However, this practice was not universal, and archivists have now recreated colour copies (https://en.wikipedia.org/wiki/Colour_recovery) of some shows where only black and white recordings survived by reconstructing the colour from these interference signals.
A very late in the signal chain color decoding. :-D