In my setup, only qbittorrent container is using the gluetun network, all other “arrs” are using bridge non-VPN connection (my ISP uses CGNAT and I have no need to access them from outside anyway). It may be my ISP does not block those indexers alternative URL’s, my previous one didn’t either though.

I am in the UK TOO, I use docker on a nas and I only have gluetun and qbittorrent using vpn, all my other containers (sonarr, radarr, bazarr, prowlarr) are kt behind vpn. What do you mean prowlarr cannot access the indexers? If you use the setting for the indexer you should be able to try different URLs for it, check if any of them works.