Yes i know of the problem of pitting extremist militias against each other (its basically what the Germans did 1919 to throw down a communist uprising just with the sides swapped)
The small advantage we have today compared to 1919 is that the Antifa isnt as centralized as former army corps, so you can choose with which group you work with, for example in the City i live in we have a kinda large and potentially violent Antifa due to their roots in football (Ultras), but their mainline is democratic. There arent many definite antidemocrats on the left side, the communist party, as an antidemocratic example from the left, got 0.1% and its mainstream party "Die Linke" got 9,8% of the Votes while the equivalent the NPD got 0.4% and the AFD, which is basically a mainstream friendly version of the NPD now got 13% of votes. So 14% of German voters are in some degree openly racist and extremist or at least okay with supporting a party which supports racist structures
I dont know of the situation in the UK, but i think you dont have such a problem with the far right as we have in germany so i assume you dont know of the problems within german justice, where the police and military - which are supposed to be the counterweight to militant extremists seeking to overthrow a state - are both full of right wing extremists, which makes sense since the German right wing values Order and Safety above basically everything else.