Let me spell it out for you.
Its a very small organisation. They found a Policeman was a member and jailed him. They found him because the entire membership list was leaked by someone who turned. If there were others at that time, they would have suffered the same consequences. Alongside that, since the organisation became proscribed they will be being heavily watched by the anti-terrorism police. I wouldn't be shocked to find out that the Police had undercover officers inside the organisation or very close, not only that but groups such as Hope Not Hate also have very active informants embedded among the fellow travellers of this and similar groups. In fact there's an ex-member who turned that provided evidence to the Police via Hope Not Hate in one of the prosecutions already secured against the members of this group.
I think it very unlikely that in such a small organisation that there will be current serving police officers. If there are officers involved in this fascist terrorist stuff, they will more likely be involved in Patriotic Alternative, a non-proscribed fascist organisation to which most of the members ran to when National Action was proscribed by the Government. Patriotic Alternative is currently the largest and most active fascist group in the country and includes some known former National Action members, PAs leader Mark Collet is thought to have once tried to take over National Action, Sam Melia a regional organiser for PA, is known to have had contact with NA after the proscribing, Jake Bewit was also in NA as was Kris Kearns from Liverpool and he was their fitness instructor, he's currently facing up to 15 years in jail for being in NA after being extradicted from Spain. If any serving police officers are active in this, it will be in Patriotic Alternative not National Action