There has been quite a bit of confusion over whether or not the British required warrants for their investigations of the suspected terrorists. The conservatives want you to believe that the Brits can listen in on or search anywhere without a warrant. The liberals want to intimate that strict warrants apply just as they have in the past in the United States.
In the interest of clarity, I went to the source. This area of surveillance is handled by the British Home Office which is somewhat the equivalent of our Justice Department. Since the passage of some anti-terrorism laws, surveillance can be conducted in the UK with a lesser standard that before, but a warrant is still required for the majority of activities. In rare exceptions purely for the purpose of anti-terrorism, search and surveillance can be done with only the signature of the Home Secretary, but the Secretary must have a presentation of particulars in the case and the information must be reviewed by a legal entity.
To put this in the U.S. context, normal warrants could be issued by any Federal judge. Anti-terrorism investigation would be allowed by the signature of a FISA court judge, but Alberto Gonzales (not an underling, not a deputy, but the man himself) could authorize these activites for a short period of time and then go to the FISA court for approval, which is exactly the way it is now.
In any case, the total investigation in Britain was done with warrants either of the formal sort or with the signature of the Home Secretary. None of this was under the authorization of Tony Blair as Prime Minister and it cannot be done by George Bush as President simply because he wants to.