If you're only licenced for automatic cars you can only drive a semi-auto in the full-auto mode. Most have both modes as a semi-auto is just an auto with a bit of driver input.
If it's anything like the PSV/PCV ie bus license then you would be able to drive it on an "automatic" license, changing gear by using an electrical switch. You only need a full license if you have to use a clutch. Double-check with DVLA just to be certain.