Shadow Home Secretary Dominic Grieve asked an important written question recently:
"Mr. Grieve: To ask the Secretary of State for the Home Department how many persons suspected of terrorism offences the Government has been unable to deport owing to (a) legal challenges and (b) an assessment that the individual concerned will be tortured if deported to the country of nationality in each year since 2001. 
Jacqui Smith [holding answer 6 November 2008]: Foreign nationals suspected of involvement in terrorism are considered for deportation action. However, deportation action cannot be taken where it is concluded that removal would be contrary to our international obligations, in particular under the European Convention on Human Rights (ECHR).
16 individuals were certified under part 4 of the Anti-terrorism, Crime and Security Act 2001 as suspected international terrorists and were detained on the basis that their removal was prevented by a point of law which related wholly or partly to an international agreement. One other person was also certified under part 4 but was detained under other powers. Those detained had the right to leave the UK at any time, two did so. Six have since been deported.
Since 2005, there have been 19 cases where deportation action on national security grounds was commenced, but was later discontinued because it was concluded that it would not be possible to demonstrate that removal would be in conformity with the UK’s international obligations, including our obligations under article 3 ECHR. These cases are kept under review.
There are currently 11 cases where we are seeking to deport individuals on grounds of national security because of their suspected involvement in terrorism. These are at various stages in the appeals process. In a twelfth case, the appeal against the decision to deport was allowed by the Special Immigration Appeals Commission as it was not satisfied that the case for deportation on national security grounds had been made out."
You don’t have to be a headbanger to be worried about this. And you don’t have to be a right wing firebrand to say that you shouldn’t expect a country that you want to destroy to be overly concerned about your welfare.
It is also important to ask whether deporting people who pose a threat to our country is the right approach, even when we are able to do it. If someone is truly dangerous – and might go away and make plans for an attack – should not they be tried here and, if found guilty, incarcerated?