Just to add something to this subject.
The withdrawal of first class accommodation from more local services is a process that's been going on a long time, at least 50 years I should think. Those who remember when first generation dmus were refurbished will recall that many of them had their first class areas "converted" to second class (as was) by nothing more than recovering the seats in the appropriate colour/style moquette. This avoided guards having conflicts with passengers on this very issue. The preferred method of dealing with it was to lock the first class areas out of use and I remember that many guards did precisely that where the stock allowed them to do so, in particular where first class was in compartments. However this is often not an option with modern day trains but it would appear nobody has thought it worthwhile clarifying the situation by way of a national edict in writing.
And yes the North Downs line, which used to run Reading to Tonbridge, is exactly where I often experienced all this!
I'm pretty sure that BR refurbished dmus were all turned out from the workshops in the late 1970s and early 1980s with their first class compartments still in place and classified as such. The seating moquette was, I seem to recall, a light brown colour, as opposed to the blue used in second class.
When removal of first class began in the 1980s, it tended to be on an area-by-area basis, such as when the PTEs in Yorkshire and the North West decided to end first class on the services they sponsored - and the initial declassification was done in a similar way to what happened on FGW/GWR more recently - by just removing window stickers and the yellow lines above the windows.
Seat moquettes were changed more gradually, as the depots had better things to do with their time. In West Yorkshire, where I lived at the time, passengers were certainly allowed to use these compartments from the day first class was abolished, whatever colour the seat covers were.
What you observed on the North Downs Line probably had more to do with the fact that the Tadpole Class 206 demu/2-EPB hybrid sets used between Reading and Tunbridge from the mid-1960s to about 1980 did not have any first class seats. The Western Region dmus that replaced them did - for Paddington commuter duties - so the guards would probably have locked the first class compartments out of use to avoid any confusion.