The CD and RA indicators are also used for FCC services at Cambridge and Ely. It is not common to see a guard on these services.
On FCC, there's CD/RA at Royston, Stevenage, Welwyn Garden City, Hertford North, Finsbury Park and Kings Cross (obviously). At Stevenage where a guard is seen regularly (East Coast/Hull Trains), the CD/RA is ignored and a bat/lamp are used instead. I suppose, technically, CD/RA could be used for these services, if the guard was in a position to see the CD/RA box, which wouldn't be an issue with Hull Trains, but may be with East Coast. This isn't the 'done thing', however.
Also, CD/RA is not used for 12-car trains with FCC (bat and flag, more than one man job), generally. There are occasions when it can be used. When 12-car trains run from platform four at Stevenage via Hertford, the driver can clearly see the CD/RA box and the platform staff can see the whole train. That is rare though.