Update. I have logged a call with tech support and sent them a few screenshots of what I get on the phone.
Last night I rebooted the phone and began the doenload of England again. Unlike France and Spain (that I will also need) England doesn't allow regions and so it's a 746MB download.
I kicked it off at about 2300 BST. When I checked at 0930 BST today it was still downloading according to the notifications area on my Motorola G50.
Soon after that changed to "Regions Downloaded". Clicking takes you to the app. Excitement builds.
Change the functional option to offline. Open the test route. It's from my house to a parking lot 2.3 miles away.
Nada. The message comes up to say that "Offline navigation enabled - No route could be calculated using your offline maps........ Go Online" - that of course works as it is using the Wifi in my house and 4G/5G on the road.
The online navigations worked perfectly. Good instructions and as I knew where we were going it was accurate.
Since returning from the test drive, I have tried again. Phone rebooted. Into Offline Maps / United Kingdom / England - click on grey download arrow.... what I assume from many visits to Amazon Music, the blue wheel going around the X ought to mean it's downloading. So far it's been doing that for around 30 minutes.
More to come?
Thanks everyone for their patience!