I would imagine part of the reason Dr Yusuf al-Qaradawi has been denied entry into the UK is because he defended suicide attacks in a BBC interview a few years ago, though they did grant him entry in 2006 (though he didn't come) so it seems they've changed their stance on him since then, maybe due to pressure from numerous politicians.
But as Oryx says, it's a government's perogative to deny entry to anyone they want, though no doubt some will see this as preventing freedom of speech.
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