Sorry Trumpy, I missed your earlier question about the antenna feeders.
I can't say that 300-ohm twin lead was never used on very early sets here, but I've worked on vintage British TV sets dating right back to the late 1940s and I've never found anything other than the Belling-Lee coaxial type of connector.
On signal leads, the main limiting factor for video signals is the attenuation and phase shift of the higher frequency components of the signal. That's determined largely by a combination of the cable's capacitance and the impedance of the source, so the total allowable length can vary considerably.
It's certainly possible to insert amplifiers to maintain signals levels on long cable runs. The principle is used extensively on long-distance telephone trunks, undersea communication cables, etc.
