So . . .if i understood well, you want to restore your old CGX 800x600 ScreenMode, right?
Your A1200 has a monochromatic CVBS output, right?
You must buy or make yourself, an CVBS>SCART cable or adapter.
In that way, you can view on your TV set the ScreenMode Prefs, and restore to CGX 800x600.
Ciao