I would have thought that a Special Forces Soldier, trusted by the army to carry weapons and ammunition on duty as a normal part of their operational day to day life; should be licensed to be able to hold some self defence weapons and ammunition at home.
However, they should be secured as would any other licensed firearms holder.
Sounds reasonable, I keep two powerful rifles and ammunition at home, in accordance with the required security conditions. As far as the Glock is concerned I'm sure the licence could be worded along the lines of; the licence covers weapons he may be required to use in accordance with his duties and as determined by his CO and may include weapons that would otherwise be prohibited by law.
Personally I think that after he had his original sentence reduce he should have, as suggested, done the grey man thing. He didn't lose his rank,(which would have made life interesting at Colchester!) and he wasn't given a disciplinary discharge, I doubt he would have continued within the SAS but he might have been able to stay in the Army, who knows? As it is I understand he is getting an administrative discharge, "Services no longer required" which does, I think, preserve his pension rights. He is possibly still acceptable to some of the private security companies.
What I don't know is whether his sentence was detention or prison, and did it carry an automatic discharge?