Bert Coules wrote:

the builders have chased in a trunking run up
to above the ceiling line and installed (though not connected) a single run
of Cat 5 from the existing front-wall junction box

I see from the photo that it is a genuine BT box.

to what will become an AV
equipment cupboard. A second Cat 5 run goes from there to the desk area.

Are they anticipating my putting the router into the AV cupboard, perhaps?
Would that be the standard thing to do?

Not exactly standard as most people don't have a media cupboard, but not
a bad place to hide it, I have my router in a cupboard with all the
other IT clutter, generally no need to see the blinkenlights.

They've used one stretch of the same trunking to run some speaker cable -
any problems with that arrangement?

Shouldn't be, DSL signals start well above audio frequencies.