Hi imagewizard,
Broadcast right forms one side of the so-called 'communication to the public right'; the other part is known as the 'making available right'. A copyright owner has three fundamental rights: the right to authorise copies to be made of his work; the right to distribute the work and the right to communicate the work to the public. In essence the last two of these cover publishing in hard copy form (on paper or vinyl etc) and publishing electronically. The electronic publication covers broadcasting at a fixed time and place, whereas the making available version means that the work can be accessed at a time and place chosen by the member of the public. In the real world, broadcasting is done by broadcasters to a wide audience all at the same time in a particular region or territory, while the making available right is a one-to one communication such as streaming, for which there are, generally speaking, no temporal or territorial boundaries.
As you have identified, none of these rights exclude the existence of underlying rights such as the copyright in a drama or musical performance. Where broadcasting etc differs from the original rights in literary, dramatic, musical and artistic works is that while the latter have a duration based on the lifetime of the author, the former (along with sound recordings) have a fixed duration which is triggered by the date of first publication or broadcast. Thus a repeat of a Dr Who episode 10 years after the original broadcast does not re-start the clock on the broadcast right.
The way this works in practice is that the author, let's say of a drama, authorises the BBC to record and broadcast his play. Once that work in that form (same cast, stage direction etc) is a broadcast, the BBC can, usually, then sell the rights of that broadcast to, say ABC in Australia, and the ABC do not need to get a separate permission from the author to use his play which is within the broadcast. The same appilies to the broadcast of a musical performance etc. Once the period of protection in the broadcast has ended (after 50 years in the UK) then the broadcast enters the public domain. However that doesn't automatically mean that anyone can publish an early Dr Who episode first aired in 1969, unless of course they had access to a BBC mastertape of the programme as it was broadcast. If you had a home recording (unlikely from 1969, but bear with me) then that would have been an infringing recording and so broadcasting or making it available now would amount to infringement even though the original copyright in the broadcast had expired. And in theory at least, someone doing that could be sued both by the BBC as broadcaster, and the author of the screenplay, music etc because their original permission to the BBC did not include the right for someone to make an illegal copy of the broadcast. In practice I don't think this sort of joint suit has ever been launched. Just for the sake of clarity, I should add that the
Copyright Designs and Patents Act 1988 provides an exception to allow the taping of programmes (TV or radio) off air for strictly personal and private use (known as time-shifting) but this exception is immediately made void if the recording is subsequently released or made available to an audience outside the person's immediate family or household.
You mention films. These are hybrid works. You would think that they were similar to broadcasts (and indeed they were once treated in much the same way). However following pressure from the Hollywood studios, the approach changed and today films are works of joint authorship, shared between the producer and the principal director. As you mention, the duration of copyright in a film is based on the lifetime of the last surviving author plus 70 years after his/her death. The music in a film is only treated separately if it has been released separately eg on a music CD. The music in the soundtrack within the film does not have a separate duration to the film itself, if, say, the composer of the music outlived the director and producer of the film. And a film within a broadcast (for instance, one which had been cut to fit neatly into the schedule, or to accommodate ad breaks) would still be a broadcast and subject to the 50 year duration, and not the full lifetime plus 70 years which applied to the cinema/movie theater version.
It's all pretty complicated and I have omitted lots of detailed stuff (satellite broadcasts, cable TV etc) in order to keep it as simple as possible. And a great deal of the actual detail in each specific case will be covered in contracts between authors and broadcasters. For instance the author of a play may only release the rights to broadcast his play within a specific territory or language, and can authorise whether the broadcast can be repeated or released on something like the BBC iPlayer or other catchup TV service. There may be other stipulations such how much editorial freedom the broadcaster has, particularly if the writer has a lot of clout. However I hope this answers your fundamental question.