Legitimacy and power, a sign of the government's control over the electorate, would be best represented by the recent anti-terrorism legislation of March 2005. The previous anti-terror laws (2000 Terrorism Act and Anti-terrorism, Crime and Security Act 2001) expired on March 15th, 2005. They, therefore, needed to be replaced by an act more relevant to the country's needs four years after 9/11.
The government, with an overwhelming majority of over 160 MP's in the Commons, got the acts of 2000 and 2001 swiftly through Parliament, primarily on the back of the aftershock of the terror outrages in America. That is the power it has in that the sheer number of democratically elected MP's could get these acts through Parliament despite the fact that some within the Commons found their powers draconian; though the opposition to the acts was nothing like the government was to face in February/March 2005.
The legitimacy for the 2000 and 2001 acts came from the fact that it was legislation that came from a government as elected by the people in 2001 via Britain's democratic electoral system whereby every 5 years registered voters can express their political will at the ballot box.
Regardless of the quirks of Britain's electoral system, it is the system that we have and we have to work with it. That system gave the current government an overwhelming parliamentary majority and in line with representative democracy, the acts were passed through the Parliamentary system and came into force in 2000 and 2001. Such a massive Parliamentary majority gives any government control in that it can drive the political agenda knowing that its proposals/bills etc. will almost certainly pass into the law books.
After September 2001, the government was in a position of high moral ground and could guarantee the public's support for any legislation targeted against terrorism, even if that legislation, once on the statute book, remains there for a while.
In February to March 2005, the government found that the power that it has can be curbed by the democratic system that exists within Westminster. The government presented to the Commons a bill that, if it was passed, so some argued, the traditional rights of the people of Britain, guaranteed by Habeas Corpus etc, would be consigned to history. The government argued that the fear of terrorism meant that such an act was needed, even though it would bring in “control orders” which, in the bill's original state, the Home Secretary would have control over. Civil rights groups saw this as the biggest threat to the nation's civil rights for 300 years. To even start to support the bill, civil rights groups stated that a judge must have the right to introduce a “control order” not a minster.
Even with its mighty majority in the Commons, the government faced a major backlash not just from the opposition but also from many in its own ranks. However, the bills passed its first reading in the Commons and moved onto the Lords. Here it was defeated and was passed back to the Commons. After several sittings in both houses that lasted well into the early hours, an act did receive the Royal Assent and became law before the 2000 and 2001 acts expired. However, the final act was not what the government, with its large parliamentary majority, had originally wanted - despite its obvious power within the Commons. A “sunset clause” and a judge imposing a “control order” were reforms the government needed to make to get support for the bill.
As a result of the democratic procedures laid down by Westminster, the 2005 Anti-terror Law does have legitimacy as it was voted for and supported by both Commons and Lords - eventually. However, the process also showed that the apparent power of a government can also be curbed if there is sufficient support in both Houses and that even with a majority of 160+ MP's, the government simply cannot railroad legislation through Parliament. BBC correspondent Andrew Marr called the whole process “a great day for democracy“.