I needed to sort out the issues with my radiator valves because this summer I will need to get all the valves changed (TRVs included this time) and didn't want any nasty surprises in terms unavailable parts. My final understanding is the following:
1. All my old radiators and modern radiators use tails which all have a 1/2 BSP male tapered threads going into the radiator.
2. Therefore, since replacement valves come with a tail, it should be possible, worst case, simply to replace the old tails (and valves) with the new tails (and valves).
3. The old standard for valves was to use a 3/4 BSP union connection ('O' ring or fibre washer seal) to the radiator tail.
4. The new standard for valves is to use a 15mm compression connector to the radiator tail.
5. Many plumber merchants don't appear to stock radiator valves with union connections any more - fortunately, other outlets, such as Screwfix do stock them still.
This has been a bit of a steep learning curve for me, not helped by not knowing what correct terminology to use at times! However, I got there in the end!