If you want an example of why getting an established broadcaster - let's go with the Beeb - would take your ideas for a documentary and make an utter hash of it, look at the fairly recent Mike Oldfield documentary that was on BBC Four. It was an hour long, covered a reasonable amount of his early life (because it was all told via interviews with himself and his sister), spent a good chunk of time talking about Tubular Bells and the immediate aftermath of that, then covered the 20+ years encompassing his singles chart success, Tubular Bells sequels and move to Ibiza and into dance music in literally the blink of an eye, then spent the remaining 15 minutes talking about his appearance at the 2012 Olympics Opening Ceremony. All of the footage was stuff that the BBC had rights to (studio live performance of TB, 2012 opening ceremony), or footage provided by himself. As a career retrospective it was woeful. It wasn't even that interesting an interview. That's what you'd get with the Beeb - minimal footage of non-BBC-held rights and huge swathes of (interesting) career glossed over.
A much better model would be for an established documentary maker to make it with private funding, with a view to selling worldwide broadcast, streaming and DVD rights. BUT...in order to do that they're going to want to tell the whole story. It has to be interesting enough to pull in the non-hardcore fans; it needs to be warts'n'all. So which ess-than-wholesome bits of The Shads story would you want them to focus on? Jet's drinking? Bruce's depression? John Rostill's death? Whatever acrimony went on around the split in 1990? Because "4 lads make it big in the charts for a few years, kickstart the careers of some guitar heroes, lose Eurovision and then release a bunch of covers albums" isn't going to spark the interest of many private investors or media outlets. I'm paraphrasing massively there, obviously, but without the "interesting" stuff there's not much of a story to tell. That's not to say that we wouldn't want to hear it, but that's why we're all on this site; it's everyone else that you need to sell it to to make it worth it. How successful was the Telstar movie? £22,343 opening weekend returns on a £1.25m budget, and that had some sex, drugs & rock'n'roll in it.