A government should be allowed to decide who they let into the country. If they want to ban a specific person or group of people that is their right. Trump is not a citizen of the United Kingdom, so the British don't owe him anything. It might make people angry, but their own visiting policies are their own business.
Yes, British government officials should be able to ban Donald Trump from the UK. While freedom of speech is a right, when it reaches a level that evokes violent ideologies and has the potential to provoke hate crimes and violence against minorities, it should not be tolerated. People have been banned from acquiring a visa to visit the UK in the past for like behavior, so Donald Trump should not be an exception and the same legislation should be exercised.
British MPs should be allowed to ban Donald Trump from the UK, because his inflammatory rhetoric could potentially incite rioting, hate crimes against Muslims, or other forms of violence. Since incitement to violence is a threat to national security, MPs are well within their rights to keep him from speaking in the UK.
British MPs should not be allowed to ban Donald Trump from entering the UK. Even though he may be extremely unpopular in the country that does not mean that he should be banned. Democracies on simple principle should not ban foreign political figures. Donald Trump is not a war criminal and should be allowed to express his opinions in any free democracy in the world.