Well known simple fact, not all terrorists are refugees or foreigners, mostly they are home grown, radicalised while travelling to Syria, Libya etc... You can't expel home grown terrorists!
You're right that you cannot expel citizens (single-nationality citizens that is, not double-nationality citizens). The "well-known fact" that you quote is however yet another lie, but even if it was true: if a citizen indeed goes abroad into a terror training camp (which is explicitly forbidden), you have the legal means to arrest them immediately upon returning. Arresting them and locking them away is not perfect, it is not a permanent solution, I am aware. But for the time being it's better than nothing. While you are locked up, your ability of hurting innocent people is limited. Alas, that isn't happening.
A better "well-known fact" than "most terror/crime/bad stuff comes from citizens" is that crime in general is registered and reported very selectively (at least in Germany, I wouldn't know about UK). When, a year ago, there was a complaint about the police in one particular city not recording tens of thousands cases of theft from a particular group of people at all, the police commisioner was questioned. His answer was: "Oh come on, they're poor people and in every case that was their first offense". When asked how he could possibly know it was their first offense since police didn't even care to verify their identities, he couldn't answer that.
Whenever a violent crime happens and it's someone from a particular group of people, you only hear "a man" on the news. Well, not always, to be fair, this week both the Syrian who stabbed his psycho counsellor, and the guy who attacked the child and its mother were "properly attributed". That was most surprising, usually it's not the case. Usually, when a foreigner with double-citizenship commits a crime, it is much emphasized "a citizen" (oh shock! a citizen! one of us!), but if a Tunisian drug dealer drives a truck through a pedestrian zone, then the fact that he is Tunisian does not play a role for the crime. That's selective news, and thus a lie.
Also, a lot of the radicalization initiates in loco by people, and with people whom you could easily expel because they are either tolerated aliens or illegal border-crossers who claim being refugees, but they are not politically pursued. Few, very few (Pierre V. being a famous example) cannot be expelled, but they could be arrested for what they are doing, and they could be denied welfare benefits. And yes, they could be held in a prison without internet access. You know, the very important, much stressed right of freedom of information that prisoners retain despite being criminals does not mean you have to have a 42'' TV in your prison cell, and it does not mean you have to have broadband internet access. A single TV in the common room for everybody perfectly satisfies your civil rights after you have turned against society. It is not necessary for a felon to have a better life than a worker, and it is not necessary to provide an arrested agitator of hate and violence with the means to continue his "work" from within prison, via internet.
But about the expel part: As I've pointed out earlier, the ever-alleged "rights" of staying that everybody who isn't a citizen allegedly has exist only if they have demonstrated that they are politically pursued, that is the explicit wording of the law. Being a foreigner from a poor country or a country where there is war is not enough. Being from a country where some people are politically pursued is not enough -- you must be pursued (and, for political reasons, not for religion or the color of your skin, or another reason). Even being in concrete danger for whatever reason is not enough. No rights, if the state decides that you are undesirable, you're gone (well, in theory). No burden of proof of anything, it's enough to say: "We don't want you here".
For humanitarian reasons, it is generally the correct thing to protect people who are genuinely fleeing from war and running for their lives anyway, even if they do not have a right to ask for this. As long as we can afford, we should definitively continue doing so. It is the right thing to do. But that's about people who are victims.
That doesn't mean we have to be stupid, it doesn't mean we must tolerate everybody, and feed everybody. If you hold hate speeches in your mosque, or if you make public statements like "kill infidels, build califate" then you have just proven that you are not a victim, you're a perpetrator. If you carry weapons, you have just proven that you are not a victim. If you are a member of an openly criminal/terroristic organization, even if it calls itself a boxing club (kidding?), the moment your organization posts videos on youtube about overthrowing the state, you're not a victim. Heck, you're not even remotely tolerable.
If you attack people (even without killing them), if you got caught stealing for the 5th time this week (that's not an exaggeration, it's what the manager in our grocer store in town told me on a day when one of our completely harmless guests started a brawl 3 meters in front of me when being caught stealing, they needed three men to subdue him), then you are, too, definitively not a victim. If you're dealing with drugs, you're not a victim, even more so if you use the revenue from your drug deals to fund a terror attack.
Those people you can arrest, those you can expel, and it should be done.
