No, I'm pretty sure they weren't actually banned from the country for their own safety.
I'm pretty sure it's not even because of their message. Sure, they're spreading a bad message. A message which no Briton wants to hear. A message coming from a group of people who are not affected by what they're angry about and shouldn't be concerned by what they're angry about and live on the opposite side of the planet from it. But that's not why.
I'm pretty sure they were banned because they spread their bad message in a bad way. A terrible, hostile, spiteful, attention-seeking way that insults entire nations, races, cultures, good Christians and offends the sensibilities of all who bear witness to their performances. They protest, attack and defame people who have absolutely no connection to the cause they're protesting against. Worse still, they offend the very concept of freedom of speech by offering the perfect example of why free speech is a bad idea.
No no no. These are truly, genuinely bad people and their cruelty and wretchedness goes far deeper than being simply anti-gay. In fact, I would go so far as to say that if there were even a remote chance that some brave Brit would butcher Phelps and his entire family there'd be at least one person in the UK government arguing that they should be allowed to exercise said free speech.