Asia is not one travel proposition but several dozen, and almost nothing practical carries across from one to another. What is gathered here is China: the mainland, with Hong Kong, Macao and Tibet.
Those are not variations on a single destination. Hong Kong and Macao each run their own immigration control, issue their own currency and set their own entry rules — so a journey between either of them and the mainland crosses a border in every sense that matters to a traveller, and the document that admits someone to one does not necessarily admit them to the next. Tibet adds a permit requirement of its own on top of a Chinese visa.
Entry rules for British citizens differ in each case, and the mainland's present arrangement carries an expiry date — which makes them the first thing to check and the first thing to go out of date.