Is there any english law similar to §89 a,b,c StGB (german law)? (its to some extend based on
S/RES/2178 (2014)). Its controversial law, because it extends the scope of criminal liability quite far beyond any action, that itself would be deemed criminal. It allows criminal punishment (up to 10 years) of people who travel to training camps, learn any kind of skills that enable them to participate in terrorist attacks or are entangled with finance of these activities (the updated law allows to sentence people for marginal cash transfers). Additionally there is §129, which punishes membership in terrorist groups. On top, these circumstances give law enforcement quite extensive competences to monitor/investigate suspects.
These laws allow the prosecution of anyone who went to Syria (to join ISIS or other organisations) even if the prosecutor can't prove any individual crimes.
I'd be surprised if there is nothing similar in english law.