Brian Sharrock wrote:
It's time to consider 'doing' the Lounge ...
snip
While I'm at that ... I might as well tackle the 'signal'
wiring that's accumulated into the house.

I'm looking for suggestions, advise, don't-do-this, from
readers for coping with the '5.1' cables; the computer network
cables; that have proliferated.

Wireless may be an alternative for eliminating the computer network wiring.
It does not work well for all houses, and is comparatively much, much harder
to work out if it will work beforehand.

Should I consider putting in green tubing to enable me to
pull-out the old stuff and push-in better/different audio cables
in the future - if I either win the Lottery or the cost of audio
kit comes down? What appropriate termination boxes are
available?

Look on http://www.screwfix.com/ for an idea of the network terminating
boxes, they have a couple.
Conduit is a really, really good idea.

You'r practically guaranteed to find out that you want to put in another cable,
be it for routing a satellite dish signal to the decoder box across the
house, to the fish tank leak alarm, to a little circuit that tells you when
the hot water is back up to temperature, to ....
And always make it at the least twice the diameter you think you need.

As to audio speaker cable, or audio signal cable, there is no perceptible
improvement to what existing professional cables can achieve.
Vendors of carbon fiber cables made only from the hair of virgins et al
are merely peddling ignorance.