Don't know what it's like in the UK but CAT5 or CAT6 ethernet cables are so cheap here
I think you're pretty much guaranteed cheap anywhere in the western world.
are interference-immune in a modern RF-smog-filled home.
Technically you need the shielded CAT5/CAT6 cables (FTP / STP / SFTP) to be fully immune .... but unless your RF smog is really bad, normal unshielded CAT5/CAT6 should be perfectly good as the wire twists inside the cable serve some of the purpose of interference immunity....