Ok, it seems that on a solo forum still solo is more or less forbidden. An ask for a list ends in a discussion you need guides. But sometimes you dont want and need guides, on most sportsdiverdives not.
UK is solodiverfriendly, great and I know. You can dive solo with kaolak scuba adventures, but I would ask in front before booking again.
In the Netherlands, you can dive everywhere without guides and solo. Northseawreckdiving can be done best and safest solo.
Malta and Gozo are solodiverfriendly, you can rent tanks and go diving.
I am a promotor of solodiving, it can be done safe but is not for everybody, that must be mentioned. I dont have any limits in depth or deco as I have full trimix and full cave certs. You can do solodive or self sufficient courses nowadays, that is also a proof it can be done safe. And divers know that. No stats that solo gives more accidents, whatch coming out of the closet from Mark Powell.
Sadly, it seems a list of solodiving friendly operators based on part of the world is hard to make. Such a list would make choosing the right or best divecenter easier.