The S20 references date from when the UK bandplan changed from the regional basis to modal basis. Once different regions of the country each had their own block of 2m but the growing use of FM and the every increasing numbers of synthesised radios meant the bandplan changed to the bottom 1MHz being for non-channelised modes (CW, SSB, beacons etc.) and the top 1MHz being for modes such as FM were channelised operation makes sense. The top 200kHZ was reserved for Space<>Earth comms.
Repeater inputs were 145.00 - 145.175 with outputs at +600kHZ known as R0-R7 (for repeater)
Then you got the simplex channels from 145.225 - 145.575 S9 to S25 (for simplex) with S20 (145.500) being the FM calling frequency. You used to hear people call CQ and the say “QSY to S16”. (145-400).
There were radios of the day that would have a 20-40 position switch to select the channels and would just show a number such as 17 such as my old Sommerkamp TS-280FM. This would equate to S17 = 145.425. It wasn’t long before radios started showing digital readouts of the some or all of the frequency, maybe just the last 3 digits and an LED for 144 or 145. If you only had a number such as 0-39 then the channel names made some sense, but it always seemed more confusing to me when by the time I was licenced in 1990, all new radios had an LED or LCD display.
I never used them personally but know a few “veteran” G8 amateurs ( issued 1964-1981 ) who still use them to this day. The introduction of 12.5kHz spacing meant they were all renamed in a confusing way considering this happened when everything had a frequency display. So S20 became V40. I can recall precisely zero people using the new references.
All of which reminds me it must be time I organised another Vintage Electric Handbag activity weekend, a chance for people to get their FT290, TR-2300, IC-22 etc. radios out of the cupboard and on the air.