I'm a lucky old sod who's got an all stations Silver 1st class pass which I'll keep for the rest of my life, and that means I don't use my priv card very often; however, the new one's validity started on 1st April and when I put it in my wallet I thought the endorsement on the front had changed, and so I looked on the back. I was surprised to see that endorsement 8 reads "Available also for First Class privilege tickets between former GWR stations"; I believe the reverse of the card is standard for 1st or Standard class travel, so are there still some people alive using ex GWR conditions?
I worked in the goods office at Port Sunlight back in the mid 1960s and there was one guy who was still on his old company terms (I THINK he was ex CLC); this gave him 1st class travel on the old Mersey Railway, so he traveled standard class from Port Sunlight to Rock Ferry, then 1st class under the river to James Street, then back to standard for the rest of his journey home from Exchange
Retired staff can buy £2 return tickets for any children - they don't have to be related - between any two points on the national network; however, they are only valid in Standard class, and so 1st class passholders have to down-grade to use the benefit. Grandchildren under the age of 5 can travel free with their grandparents in 1st class; one of my grandchildren looked much older than he was when aged only 4, and so we sometimes got enquiring looks from staff who were checking tickets!
There is quite a lot of inter-availability between Network Rail and London Transport lines in the capital, and sometimes it's quite odd; since March 2020, a map of the London rail network has been available which shows it all - thankfully! I remember having quite a long argument at Paddington about five years ago, and nobody on the gate line - or their manager - knew that a BR all stations pass was valid on the Bakerloo Line between there and Queen's Park. TfL Rail, Overground, and the Elizabeth Line - when it eventually opens - are part of the National Rail network, and so are automatically covered by passes/privs, but there was an odd-ball (now corrected) where it was possible to travel on the Jubilee Line between Stratford and Canning Town but not on Docklands between those points, even though Docklands replaced a former National Rail service (Docklands can now be used between Stratford and Custom House, but you can only board/alight at West Ham and Canning Town intermediately). Another odd one is on the District/Circle line between Paddington and Notting Hill Gate; problem is you can't leave a train at Notting Hill Gate, so you have to take the Central Line to White City or beyond!
Before dated travel cards were introduced, I once worked with a guy who planned his year's travel out and then booked his allocation as singles; I think at that time he was entitled to ten free return journeys, and so his twenty singles would start of with, say, one from Wick to Penzance, then a second one would be Penzance to Scarborough, then the third from Scarborough to Pembroke Dock, and so on. Every one would be for a route which would pass through Derby (where he lived), and he would start his travels off by buying a priv to his starting point, then use his first single as far back as Derby; then the second trip he would use the remaining part of his first one as far as Penzance, then his second single back as far as Derby, and so on.
He was single and at least one of his elderly parents was still alive at the time that travel cards were introduced; I was speaking to him one Monday and I asked him if he'd been anywhere over the weekend, and he replied something like "only to Swindon to see my family". I asked if he went via London or Cheltenham, and he replied "Pwhelli"; I replied with something like "pardon?" and he explained his trip.
Priv travel cards start their validity at 10pm on the night before the date shown on the front, and at that time there was still a late train from Derby to Crewe; however, this left Derby before 10pm, and so he bought a priv to get his as far as the first station reached after 10, and used his travel card from thereon. At Crewe, he knew that the first train to Shrewsbury would be unlocked, and so he got in and snuggled down for a bit of a sleep, and at Shrewsbury he got the first train down the Cambrian to Pwhelli; he stayed on that, and came straight back to Shrewsbury. I then asked if he went on to Swindon via the Salop & Hereford to Newport, or via Birmingham and Cheltenham, and he replied "the Central Wales to Swansea!" I think he made a detour via Bristol on his way from Swansea to Swindon, but he returned to Derby the easy way via Cheltenham on the Sunday night.
As in #3, I also went on one of the Eurostar test runs, in my case with my late wife to Brussels; we went out Standard, back 1st, and we ate and drank for the whole of the return journey. In the early days of Eurostar it was worth paying the supplement to travel first class as the food offering was really good; not just a take-it-or-leave-it-tray like now.
Regarding continental free travel, there used to be a travel company called Martin Rooks (later Panorama) who provided holidays specifically for BR staff so that they could use their travel concessions; in the days before budget airlines, it was a tremendous benefit - my wife and I went with them to Schladming in Austria in September 1973, and at that time she was still employed by BR and so got free travel in her own right rather than as my spouse, and so the total travel cost was something like £2.50 each - £1.25 for the Port Tax tickets each way between Dover and Ostend! We travelled in couchettes between Ostend and Salzburg, and we were then supposed to take a train to Bischofshofen and then another on to Schladming, but OBB had learned that there were a number of BR staff making the journey between Salzburg and Schladming and provided a coach just for us which they shunted between trains at Bischofshofen.
I don't know what the current position is regarding Heritage Railways, but I do know that when Alan Garraway was GM of the Festiniog Railway back in the 1960s, he had a BR priv card and so some sort of reciprocal arrangement must have existed at that time