The first way NOT to impress is to call the British Home Secretary Jacqui Smith "lunatic ... Home Secretary of England". That in itself could be classified offensive under UK anti-hate laws.
There is no such thing in the UK as the first amendment, something conservatives in particular love to hide behind. If you call someone a lunatic quite publicly and openly, as he has done, you have to have something to back it up, you can't just claim freedom of speech as an excuse. Considering that he's wanting to sue for defamation, yet HE was the one calling HER a lunatic, this is going to be somewhat of an uphill legal battle.
And as Media Matters have reported, Savage has called an alleged rape victim "Durham dirt-bag" and "dirty, verminous black stripper", both things that under United Kingdom law are offensive libelous statements.
The relevant legislation in the UK is Incitement to Hatred laws enacted after the attacks of 9/11. It covers a wide variety of different things, from religious beliefs to homosexuality. In UK law, if you can be proven to have spoken in any way contrary to these incitement laws, you can be banned from entering the country. Mr Savage falls into the normal trap of talk radio, that Rush Limbaugh and others fall into; that they can basically say what they want, when they want, under freedom of speech laws in the US; well, that may be so - but US laws aren't UK laws. If you want to enter the UK, you have to meet the UK definition of what's allowable and what's not.
Additionally, there is the Public Order act of 1986, section 5 of which states:
[(1) A person is guilty of an offence if, with intent to cause a person harassment, alarm or distress, he—
(a) uses threatening, abusive or insulting words or behaviour, or disorderly behaviour, or
(b) displays any writing, sign or other visible representation which is threatening, abusive or insulting,
thereby causing that or another person harassment, alarm or distress.
Part A of this clearly covers what Mr Savage said that Media Matters reported, and his problem challenging this in law is that if you have committed and offense which is punishable as a criminal offense by UK laws then under the immigration act you can legitimately be refused entry to the country.
It remains to be seen if Mr Savage's legal action will succeed. With the knowledge I have of UK laws I can't honestly see it doing so. Perhaps, as a gesture of good faith - and assuming he apologizes for what he called her - the Home Secretary, as a gesture of good faith, could ask the border services part of immigration to make an exception...
But quite frankly, I don't think he'll find the way he runs his mouth that she'll be in any mood to.