Actually, in the UK context, the idea of excluding citizens is not a novel one.
Up to 1948, there were just British Subjects, and all British Subjects had the right of entry and abode in the UK.
Then, in 1948, it was decided that independent member states of the Commonwealth would have their own citizenship, with the unifying status of “British Subject” for all those who held any Commonwealth citizenship. So you had Canadian citizenship, Australian citizenship, etc. And one of the citizenships created under this regime was “Citizen of the United Kingdom and Colonies”, which included not only people from the UK, but people from all parts of the Commonwealth which were not yet independent, and did not have their own citizenships. As more countries became independent, CUKCs from those countries (mostly) lost CUKC status, and acquired the citizenship of the newly-independent country. As before, all CUKCs had a right of entry and abode in the UK.
Some time around the early 1970s, though, this changed. Concerned about immigration by, you know, people with black hair and brown eyes, the British introduced rules under which the right of entry and abode in the UK depended on “patriality”; being born in the UK, or having a parent born there. The upshot was that a large number of CUKCs no longer had the right of entry and abode in the UK. (At the same time, some Commonwealth citizens who were not CUKCs retained this right, because they were patrials.)
Matters continued until 1981, when legislative reform subdivided the CUKC category into a number of distinct citizenships; British Citizen, British Overseas Territory Citizen, British Overseas Citizen, British National Overseas, British Subject, British Protected Person. These were all UK citizenships in that they were all conferred by the UK government, they all engaged the diplomatic protection of the UK government, and they were all recognised by other countries as UK citizenships. But only one of them carried a right of entry and abode in the UK.
There have been further legislative tweakings since, and some of these categories of citizenship are closed to new entrants, and so will eventually disappear. But the fact remains that there are categories of citizenship conferred by the UK government which do not give any right to enter or live in the UK. So, as far as UK law and practice is concerned, there is nothing inherent in the notion of citizenship which involves a right of entry or a right of abode.
Denying a right of entry to a citizen on the basis of what he is thought to have done is novel. But denying a right of entry to a citizen is already a long-established practice.
In post #3 Claverhouse points to the Universal Declaration of Human Rights, and the clear tension between what it says and what the British government does. But the UDHR does not have the force of law in Britain, and the tension has existed for a long time. The British government have obviously decided that they can live with it.
(The UK is also one of the few western countries to have practiced internal exile, like the Soviets used to do. But that’s another story.)