Thank you, Crimliar but oh dear! Opinion is divided then!
MS think that gethelp.exe (whilst it executes locally on one's machine) needs to communicate with their servers to do its work. Without IP4 or IP6 connectivity further down the network stack (at the transport layer perhaps) enabling communication back and forth to the application layer, gethelp.exe returns the error page I'm experiencing saying it "can't access the app".
I thought it must be a bit like MSEdge or Outlook which, undeniably, execute locally but are not fully functional without the underlying network stack routing IP packets correctly and complain when they don't get the expected response to their comms attempts.
Certainly, MS's explanation seems to me to make sense and accounts for the behaviour I see on my Win11 device but they could be wrong and, as Crimliar implies, gethelp.exe doesn't actually need real-time access to their servers to do its work. My disabling IP6 causing it to work is just a co-incidence (or the influence of a evil spirit possessing my machine 😀).
Trying to resolve this dichotomy drove me, reluctantly, to various AI apps. They all seem to agree (or are subject to the same hallucination). The gist of their opinions is that gethelp.exe needs to communicate with MS servers or it returns an error (the same one I get) and BT have not finished rolling out IP6 across their infrastructure. There are, according to the AIs, various UK geographic areas currently unable to fully route IP6. It turns out I live in one of them. Who knew! AI's universal recommendation is to disable IP6 and use IP4. Which is exactly what seems to work for me.
