All-union production died the death for ITV in the 80s.

AFAIK it did so for the Beeb too. Unions not allowed to "black" shows thanks to the Tatcher anti-union legislation.

But yes Tom, those are the routes. However given that a fair bit could be archive, talking heads are cheap and a fair dollop of the musicians will be amateurs, I'd have guessed that a 5-way co-production (A US PBS station, maybe NDR from Germany or something Canadian, UK and either NZ or Aus) could get a lot of its budget out of subsidy.