[Richard Crowley]

"Chris Siz" wrote ...
I am in the UK and have a 10m length of some cheap TV aerial coax. It
came from a discount store as a TV coax extension cable. I measure the
cable diameter as 4.8mm.

(1) Is it ok to use this sort of coax for a microphone?

Not enough details to answer your question adequately.
First of all, the kinds of microphones used for audio
applications (reinforcement, recording, etc.) are usually
balanced and require cable with two inner wires and an
outside shield/screen. If you are trying to use a balanced
mic, then the cable is unsuitable because it is unbalanced.

So, the biggest question is WHAT microphone?
Equally helpful would be to reveal what it plugs in to.

I don't want an impaired audio signal.

"impairment" of the audio signal is unlikely to occur,
and is probably not an issue here.

The length I need to use is 3 to 4 metres.

Not really important to the question. It wouldn't make
much difference if it were 1 m or 1Km

Note that even if you need unbalanced cable for your
unidentified microphone, typical RF/antenna cable is
very poorly suited for terminating in typical audio-
type connectors. It is frequently aluminum and
unsuitable for any kind of soldering, etc.

(2) Is it ok for UHF TV or is it actually quite low grade
coax and prone to interference or mess up the aerial signal?

Insufficient details to answer that question, also.
There are hundreds of different kinds of antenna-
type (RF) coax. Some of it might be perfect for
UHF TV (whatever that means in your area?)
And other kinds may be completely a disaster.

Again, "interference" is not the most likely mode
of failure of using the wrong kind of cable.