They're not prohibited per se, not by name anyway, they're 12(2) or 12(3) usually. It is possible to make one that is non-restricted though, I have an L98A1 cadet rifle which is non-restricted and it could legally be converted to semi-automatic using an L85A2 upper (which is essentially what they did with the new L98A2 cadet rifle).
But this is no minor undertaking. The number of cadet rifles in private hands is maybe a couple of dozen worldwide.
There were a few L85A1s sold in Switzerland with L98A1 lowers back in the mid-1980s, those would be non-restricted if you could find one.
It's getting hold of the parts that's tricky, I'm not sure where people are getting their parts from as the MoD has a policy of not selling firearms anymore, they destroy them. I assume they must be coming from Jamaica (the JDF used to use it, they seem to have gone to the M16A2 now) or from dealers who had dealer samples. I know a dealer in London had a fair few that he used as prop guns.