I had the same issue with another school with old version of Sophos where SEP installer was unable to uninstall it.
After speaking to support, their 'official' answer was that they will 'look into this & add support for other versions in the future' but it has been more than 4 months and I'm still waiting.
In the meanwhile, we've done the manual uninstall for now, which is not ideal, but at least my project is moving forward with installing SEP on these clients.