I cannot find the TwoNav UK Ordnance Survey maps on Google Play. Do additional maps have to be downloaded from CompeGPS using the browser on my phone?

I only need to use Ordnance Survey maps on Android. I don't have a Windows or Macintosh PC (I don't want or need one) so I have no interest in using TwoNav on these legacy platforms which I assume the CompeGPS Downloader is for.

How do I download and install an Ordnance Survey map on my smartphone when I am at a new location?