If our parliament are REALLY smart, if they debate this, they should instead offer a formal invite from the British Government for a visit mid campaign and roll out the red carpet rather than ban him. Unlike other hate speakers, he's in a different position where he has a mass audience through television. And threaten to encourage a boycott of the Trump brand until he responses to his summons and gets his backside over to blighty.
The key bit is, everyone has to be ultra nice. We protest by being nice to him not with placards.
Boris can show him the sights of London. Boris is a good diplomat when he wants to be and will give him a good run for his money if he comes up with crap. Whether you like Boris or not, he is the man for this job. He will go down well in white republican US with his stereotypical bumbling British persona as it plays into the myth of the British and the Royal Family and potential Trump voters seem to like fairy tales. Oh and take him somewhere where the local MP is Muslim too, and they can say hi as well and ask why they wouldn't be allowed to visit to US.
I'm sure that it would make for good television.