It depends why they are in Britain in the first place.
If they were born British, they cannot be deported no matter what they do. As I understand it, very few of them are British born, though.
If they have obtained British citizenship, it can be revoked, as long as they can resume their old citizenship (it can't be revoked if they would become stateless)
If they don't have British citizenship, they can be deported.
However, no one can be deported to their home country if that would place them at serious risk. That's a loophole exploited by many of these extremists because their own governments would likely kill or torture them. It's also exploited by common criminals, for example there was a Somali man who entered Britain on a forged Kenyan passport in the early 90s. He raped a 13 year old girl and was sent to prison. When his sentence ran out in 2003, the government began deportation procedures. Because Somalia was so dangerous, a court ruled they couldn't legally deport him, and he was awarded £50,000 compensation for the time he spent in prison awaiting deportation.
Crazy, isn't it?
The questioner asked why the Brown government had not deported these Imams if they had the ability to do so, I responded that I was unsure they had the power to deport them if they were simply preaching Islamic teaching even if it involved cursing the "Kuffar" and their rulers, and calling for the end of democracy and the "struggle" that would establish the caliphate.
Depends exactly what they are calling for. There's no law in the UK against cursing rulers, thankfully (I used to curse Blair almost every day). Calling for an end to democracy or the setting up of an Islamic state isn't illegal either.
Calling for violence is, and they can certainly be deported for that, subject to citizenship/ safety of their home countries.
As an example:
http://news.scotsman.com/topics.cfm?tid=1312&id=819212007