I think it depends.
Worldline (ATOS) do not use any ‘professional’ voiceover artists for their station announcements. Anne worked for ATOS. All the current male ATOS voices work (or worked) for TOCs (the same for Ruth too, but her voice actually pre-dates the ATOS system).
For on board, it is more varied and I assume the TOCs generally ‘own’ whichever recordings they use, so if Avanti wanted David Attenborough’s voice and he was up for making the recordings, then probably. Worth noting though that announcers will need to make new recordings as new stations open etc so it may not be advisable going for someone in their 90s!
I think TPE have always used staff for the on board announcements who had won competitions to do so.
The problem with using staff members is that it simply does not sound as good as using professional voice over artists.
I actually quite like Ruth Mitchell and Eryl Jones as they have quite good voices for announcements. But all of the other various Worldline announcers have not been that good. I am not really a fan of, Anne Jermy, Matthew Streeton, Karen Cunningham, Juliet Donnache, Andrew Smith, SE Rob, GW Rodger, who are all staff members.
All six of the FTPE announcers that have been used over the years, Angie Hunt, Caroline Josephs, Kath Donald, Neil Rudd, Rob Radnell, Rob Wedekind, which are all just staff members have also been quite poor too. FTPE put no effort in to their announcements. Their announcements have always been quite poor.
Compare this to the Amey (Ditra) system which uses processional voiceover artists, Celia Drummond, Phil Sayer, Emma Hignett, "Only Woman", Pauline Cavilla, SE Roger, who have all sounded brilliant. When you use professional voice over artists you get good quality announcements that sound good. But these days train companies just want to save money and ask staff members to volunteer to record announcements for free.
Interesting to hear that Ruth pre dates the Atos system? What system was Ruth used on before the Atos system?