If you have any evidence, then wait until they return, put them on trial and then lock them up.

What if you can't? What if a person travelling to Syria isn't idiotic enough to use social media or appear in propaganda videos like the ones we've seen?

Let's say we know that someone has gone to Syria to fight, we know that the person has likely been involved in terrorism, war crimes and/or crimes against humanity based on the group he/she joined and the area they were operating in.

How do we prove that? If we don't have witnesses or hard evidence we can use and they return to Britain, we just let them go? We then have potentially hundreds of trained terrorists with all the connections they need, some of who are bound to be on orders to be sent back to commit terrorist attacks against their home country, living amongst us. What do we do when one of those people kills a bunch of us? Do we say "worth it" because we didn't lower ourselves to stripping them of citizenship whilst they were away?

Let's say we can prove some of it, and we lock them up, how long are they going to get? A few years? Do you think those people will be less, or more, radicalised after their time in prison? Do you think they'll spend their time keeping to themselves and paying penance or do you think they'll devote their time to radicalising others? What happens after they're released? (See above)

I'll be honest with you, I'm usually very uncomfortable with allowing governments to take extraordinary extra powers, like revoking citizenship, and in an ideal world I'd be parroting exactly what you just said. But from my point of view you HAVE to be able to answer the questions I've put to you in order to hold the position you hold, there are some who say that because terrorism kills so few people in the UK (relatively), it's a price we have to pay for freedom, I don't agree with them but at least they're putting it on the line, anything less than that just seems pretty hollow to me.