Sort of a follow-up question to one of my previous questions.
The idea is that the UK does not recognize the Islamic State as a country, therefore they cannot just revoke Shamina Begum's citizenship by saying she's a citizen of IS. From the UK's perspective, IS isn't a country, it isn't legally allowed to any territory, any legal documents it issues are worthless, and IS "citizens" have no right to travel to the UK.
If this is the case, then presumably what applies to IS ought to also apply to Taiwan. Since the UK does not recognize Taiwan, how is it still possible for Taiwanese people to travel to the UK? The same question goes for any of several non-recognized countries in the world, such as Kosovo.
There is some detail on this question on the Wikipedia page on Taiwan passport and Kosovan passport, but it doesn't answer the question. For example it says it is possible to travel to e.g. China on a Kosovan passport even though China does not recognize Kosovo, but it doesn't explain how that is possible. How come these travelers aren't stopped at the border?