That's a great read. I knew a little about the history of Boosey & Hawkes / Besson, but certainly not going into that length. Very interesting to hear about the issues with productions and cost.
Regardless, I wouldn't purchase a Besson / B & H euphonium unplayed or at least unseen. With the issues Besson had in the late 90's and early 00's, you never know if you are going to get a gem, or a dog. In my limited experience on euphonium, I have owned 3 Sovereigns (plus I had a little bit of experience playing on German Sovereigns). One was a B & H, which played very nice. Free blowing, good intonation (for a Sovereign), nice tone and solid build quality. The other two were Besson Sovereigns (one from the 80's - can't remember the exact year and a GS model from '99). The older Besson had a warm tone (darker then the B & H), but it was very stuffy and it had an unplayable high concert B. Slightly better intonation though. The GS model was from the lottery grant period, and it showed. Bad build quality when it came it the valves and slides, which was a pity because the Sovereign played beautifully. Good intonation, nice tone, and very free blowing. Awesome projection.
So the point I'm making here is every Sovereign (actually every Besson), is different in some way. There are some gems out there that some people will take any day of the week over the a new Sovereign or Prestige. However, there are some dogs out there as well.
In reference to the particular one advertised, it looks like a great instrument. If the owner has bothered to keep it in great condition, then it might be a great Sovereign.