I do not have a full list, but you can see how the solution works on Secure Meters’ webpage: https://www.securemeters.com/index.php/products/residential/utility-retailers/smart-metering-gas/secure-smso/
I suspect we are not given much visibility on who our energy suppliers use to maintain our meters (i.e. act as SMSO on their behalf). Having said that, an easier route for them is to use the manufacturer’s own managed services (provided they provide such service). I assume this issue might eventually go away when old meters are finally transitioned to DCC. Even after this final enrolment, I am not entirely sure if SMSOs will still play a part though.