In a liberal democracy we value individual freedoms, the right to freedom of expression, religious beliefs, eccentricity and so on. Unfortunately the right of one to individual expression sometimes conflicts with the rights of others. Some even interpret their rights in such a way as to bring them into conflict with the concept of liberal democracy itself. So we have rules, often unwritten, but sometimes enshrined in law, that people should exercise their rights only so long as they do not unduly impact on other people and are not contrary to public good (rightly or wrongly usually enshrined in public policy). It surely therefore it is never possible to argue ex principe that the freedom of expression is absolute.
In the case of the Niqab, I am happy to allow a woman's right to wear a Niqab (or man, because there are documented cases of men using the Niqab and Burkha to avoid detection by the police or security forces) in private, or in public places where there is no over-riding public interest. Of course a woman can freely choose to separate herself from the society she moves in but she must accept the consequences as well, which may mean not having access to educational facilities or not being allowed into a bank, a public building or other location where security is a priority. This is a right of British society to expect social norms such as showing your face when engaging with other people, something essential for proper conversations.
In the case of Saudi Arabia, or other very conservative Muslim societies, I think it is fair for the West to challenge systematic imposition of practices that are surely anathema to civilised, developed, societies. Women is Saudi Arabia should be free to wear the Niqb if they want to, and some may do so for cultural reasons, but they should not required to by society, or by their fathers, husbands and brothers.
Incidentally, we are not consistent in applying the "woman's right to wear what she wants". Nudity and semi-nakedness, page 3 of some tabloids apart, are not normally acceptable in this country in most social situations.