Brussels Journal quotes this letter to Wilders stating that he is excluded from Britain:
Dear Mr Wilders
The purpose of this letter is to inform you that the Secretary of State is of the view that your presence in the UK would pose a genuine, present and sufficiently serious threat to one of the fundamental interests of society. The Secretary of State is satisfied that your statements about Muslims and their beliefs, as expressed in your film Fitna and elsewhere, would threaten community harmony and therefore public security in the UK.
You are advised that should you travel to the UK and seek admission an Immigration Officer will take into account the Secretary of State's view. If, in accordance with regulation 21 of the immigration (European Economic Area) Regulations 2006, the Immigration Officer is statisfied that your exclusion is justified on grounds of public policy and/or public security, you will be refused admission to the UK under regulation 19. You would have a right of appeal against any refusal of admission, exercisable from outside the UK.
Irving N. Jones
On behalf of the Secretary of State for the Home Department
There's more at Brussels Journal about the story, which has an internal British Parliamentary political dimension.
I definitely do not support the British decision, and I would oppose any such decision by the U.S. government (not that I know of any in the wind). But I should note, for those who are interested in comparing European law and American law on free speech (as I sometimes do), that the American precedent on this question, Kleindienst v. Mandel (1972), generally allows the U.S. government to exclude speakers based on their political views.