I have a full auto license so category A with code 78 and know that I can ride learner
legal 125 manuals with L plates/no pillion/motorways.

But I also have category k & P on my license, with no code next to them. Now code P is for mopeds
50cc limited to 30mph so does that mean I can ride a manual 50cc without L plates thats restricted
or only capable of 30mph?

Looks like it.
You could probably also ride any 50cc which was manufactured before the 30mph restriction came into effect...