Shotgun cartridges are not controlled in the same way as cartridges for FAC weapons. It is perfectly legal to possess and carry shotgun cartridges (excluding people who are banned from owning them due to being prohibited persons under the terms of the firearms licencing act) whether you hold a SGC or not. Gauge of cartridge is not considered either. If you are buying shotgun cartridges from an RFD, other than at a registered clay shoot, then you are required to show a valid SGC. If you are buying them at a clay shoot then no SGC is required. You can buy as many as you like and are free to take them home with you.
So the answer is "yes, you can"