It's entirely lawful to expel someone from a party for not agreeing with their policies, even if that position is a protected belief (like almost every belief is except literal hate speech.)
It's not true to say that almost every belief is protected. The EHRC's guidance states:
"For a philosophical belief to be protected under the Act it must:
• be genuinely held
• be a belief and not just an opinion or viewpoint based on the present state of information available
• be about a weighty and substantial aspect of human life and behaviour
• attain a certain level of cogency, seriousness, cohesion and importance, and
• be worthy of respect in a democratic society, not incompatible with human dignity and not in conflict with fundamental rights of others.
For example, Holocaust denial, or the belief in racial superiority are not protected. Beliefs such as humanism, pacifism, vegetarianism and the belief in man-made climate change are all protected."
I'm also guessing, based on the guidance above, that a belief in astrology or flat-earthism wouldn't be protected.