I oppose the aims of the group islam4uk which want sharia law for the UK, in fact I'm not even a muslim, I'm a christian. However I also happened to think it was wrong to ban the organisation. However I know that my view, and the view of the current government, diverge on this one.
However I hope you would agree that I should be free to campaign against the ban, if I wanted to.
However had I wanted to campaign against the ban I would have been in breach of the above act because:
(1) A person commits an offence if—
(a) he invites support for a proscribed organisation, and
(b)the support is not, or is not restricted to, the provision of money or other property (within the meaning of section 15).
(2)A person commits an offence if he arranges, manages or assists in arranging or managing a meeting which he knows is—
(a)to support a proscribed organisation,
(b)to further the activities of a proscribed organisation, or
(c)to be addressed by a person who belongs or professes to belong to a proscribed organisation.
So, had I carried a placard saying: "Support free speech, don't ban islam4uk" I would, as far as I can tell, have been breaking the law.