I tried the GPO, but it didn't work. At least not on its own.
From the description, it sounds as if I was in the same state as Mary when I tried it. Both machines had Windows 11 downloaded and were just waiting for reboot to install; On one machine I had tried out Windows 11, and reverted back to Windows 10, but it had downloaded Windows 11 again and was just waiting for me to reboot so it could install it. And on the second machine, I wasn't even given an option. It had decided I should have Windows 11, and had downloaded it and waited for a reboot.
If you are in this state, both the GPO and the Registry hack are ineffective, because Windows Update first checks if there are any incomplete updates to download in the C:\Windows\SoftwareDistribution\Download\ directory. The folder named '29915d28b757d79214ddd971f3fd6134' is the one for the Windows 11 update.
I found that if I just deleted the folder, then Windows Update re-created it again immediately. So to do it properly, you have to stop the Windows Update Server (wuauserv) and the transfer service (BITS).
At this point the Registry hack works. The GPO might also work.
Full instructions on page 5.