Thanks, Paul, for the recommendation about Combe Gibbet and Walbury Hill. Someone else had mentioned these to me as places worth visiting and now you have reminded me, I must go.
I remember the story of the 1977 hijacking of the feed to the Hannington TV transmitter. I'm sure it would be more difficult to do now, but that was in the analogue era and the technology was fairly simple. I missed seeing the actual broadcast, because at that time, we used the Crystal Palace transmitter. There is a youtube video of the prank Here
The main TV transmitters are all located at high elevations for maxiumum coverage, so provided they are accessible (as you say in the case of Hannington, there is a car park nearby), they must be good locations for visual coverage too.