Well, a very happy internet birthday to both of you for 19 days ago.
In other news, I did eventually contact LG.
Scott, whoever he is, confirmed that there is not a utility or built in way to accurately determine the network speed at an LG TV. He also stated that the network hardware is "not advanced as PC hardware and this does bottleneck speeds to the TV".
He had a peculiar idea that domestic network speeds are unlikely to reach the 155 mbps that my laptop reported. Fair enough, I don't know where he works. Maybe wherever he is, that sounds implausible. He also answered my query as though I was talking about wireless connections, which, as I talked throughout about connecting with a patch cable, didn't fill me with confidence in his attention to detail.
What annoyed me most was "TVs are designed for non commercial use and also are designed to be TVs". It's obviously (patronisingly) true, but this particular set is marketed as being up to handling a network stream reliably for 4K resolution video. What's more, since it came with an eye-watering price tag that hoovered up a big chunk of my pension pot, I expect it to deliver on that marketing.
I was so annoyed by this that I pressed the "not satisfied with reply" button which pulled up a text box to respond. But fittingly this feature refused to send after I had typed a reply.
EDITED: 20 Jan 2017 18:49 by WILLIAMA