Apple TV keeps buffering/freezing

My Apple TV 4K keeps buffering.

Does it matter what type of content you are streaming? I.e. which content provider or which app?

Did you measure the current internet bandwidth available to the Apple TV box, as measured on the Apple TV box itself? E.g. using the Speedtest app for Apple TV? What values do you get?

… the only thing I haven't tried is to UNPLUG the Apple TV and the TV itself as this would entail removing the TV for the TV wall which is a PITA because of where it is situated. Instead I have just switched off the power to the TV wall by using the remote switch. Would this make any difference?

If you mean switching an on-off switch for an extension power cable, or similar, then that is effectively the same.

If you mean by flipping a circuit breaker switch on the applicable group, then that is effectively the same.

The engineer did suggest connecting the Apple TV to the router with an Ethernet "extender" which apparently routes the signal through the electric power cable - sounds like the dark arts to me but I am not even sure that the TV wall and router are on the same ring main??

Ethernet signals over the power grid does really work, as weird as it may sound at first. But there are some caveats. I.e. do your homework (read up on how it works, what can cause issues, troubleshooting, etc.). It may not be a solution for every house.

Six tips to improve the network speed using a powerline network - MakeUseOf

A proper Wi-Fi mesh network with strategically placed nodes should make power line networks unnecessary, though. Use one or the other. It often doesn’t make much sense to use both.

I downloaded the wifi speed checker for the AppleTV as suggested and it showed a speed of 60mbps compared with 150mbps for all other devices in the house. Could this be an issue with the AppleTV hardware?

While a remarkable difference, it should still be plenty for streaming services (25 Mbps for 4K HDR, 15 Mbps for 4K SDR, 8 Mbps for HD 1080p SDR, 6 Mbps for 720p).

It seems to affect all UK terrestrial TV apps, BBC iPlayer, ITVx, Channel 4 etc.

Not a single service only, so the issue is probably not related to the content provider. I can’t think of a possible reason for just this category of UK linear TV networks.

How about Apple content, like Apple TV+, an iTunes Store movie, or a recent Apple Event in the Apple TV app? Does that have the same symptoms?

If some services do better, then the network itself seems sufficient.

If it appears time-of-day dependent, then I would suspect neighboring networks interfering. Start by restarting your router, your Apple TV box, and any optional other network devices in your home network (boosters, mesh nodes, switch, etc.).