Can a non-Islamic government or court decide who is a Muslim?

When, in the 1970s, Ahmadis approached the South Africa High Court seeking to protect their human rights and sought a declaration that they are Muslims, the Sunnis, after failing to have the case dismissed on technical legal reasons, withdrew from the case using the excuse that a non-Muslim court of government does not have the right to decide who is, and who is not, a Muslim.

To our great astonishment we now find Sunnis in the UK demanding that the British government declare Ahmadis to be non-Muslims. The reason is that in front of a Western audience they have to hide behind Lahore-Ahmadiyya beliefs and hide their own.

Please tell us Sunni friends what has changed that now allows a non-Muslim government to decide who is, and who is not, a Muslim?