Actually, such a law can be implemented without unduly restricting criticism and mockery of religion. The law you speak of is actually modeled on Canada's long-standing hate speech laws, which have been more or less successful at curbing extremist rhetoric while allowing critics and comics to do their thing. The Guardian article was a bit disingenious as none of the comedic examples given in that article would be caught by the law; it's targeted at statements of sheer hatred as might be expressed by the secular fanatics at the BNP or by religious zealots who promote the blowing up of churches, synagogues, etc.
Here's what the imam at the Cambridge mosque had to say about it:
http://www.bbc.co.uk/religion/programmes/thought/documents/t20040708.shtml
(He's also a professor at the university.)