If a BA Gold or Silver is flying on a BA-coded flight operated by Comair, they have lounge access to the SLOW lounge by virtue of their status. It really is as simple as that. It's a BA flight, and for this purpose that lounge is the BA lounge. There are plenty of us who are doing this multiple times a year.

There are some other circumstances in which a BA Gold or Silver might also get lounge access, but that is immaterial if you're flying on a BA-coded flight.