Now, this bit's a bit wooly. How certain are we that it is "illegal" to fight in Syria and Iraq? Do we have specific laws in the UK or under the Geneva Convention or something that define this? Furthermore - you need evidence to prove that someone has committed a war crime. Being in ISIS is not necessarily a war crime in itself.
A lot of german Wehrmacht and even SS volunteer soldiers were largely unaware of the scale of atrocities they would commit, at the time they joined. Their action of joining, therefore, cannot be considered a warcrime. Their personal actions in the war, however, can.
Now there's a difference between the german volunteers, and IS voluonteers. The latter know darn well, that IS stands for wiping their ass with Geneva conventions, and every possible human right there is. So I would consider the action of leaving their country, with the intent to join iS, a warcrime, the minute they enter the airport / canal tunnel to take the plane / train going in that general direction.
There is a difference between fighting for ISIS and having an intent to commit a terrorist attack in Britain. Let's not get carried away here.
Is there? From all I've seen and heard of IS spokespersons, they intend to spread their califate over the entire world. Just not right now, cause they're too busy putting heads on spikes in Iraq.
If any government starts deciding what ideologies are to be proscribed and what ideologies promoted then I think we are in serious trouble - more so than from ISIS.
We did this long ago, when we signed the Geneva conventions, and human rghts declarations.
But that aside, yes, we should always be wary. Quis custodiet ipsos custodes?