Brits' House of Lords Rule Against Indefinite Detention

From remarks in the judgement by the House of Lords that the British government is wrong to detain foreign terrorist suspects indefinitely without trial, via author William Gibson's blog and a Guardian article:

Lord Hoffman:

This is a nation which has been tested in adversity, which has survived physical destruction and catastrophic loss of life. I do not underestimate the ability of fanatical groups of terrorists to kill and destroy, but they do not threaten the life of the nation. Whether we would survive Hitler hung in the balance, but there is no doubt that we shall survive Al-Qaeda. The Spanish people have not said that what happened in Madrid, hideous crime as it was, threatened the life of their nation. Their legendary pride would not allow it. Terrorist violence, serious as it is, does not threaten our institutions of government or our existence as a civil community ...

Such a power in any form is not compatible with our constitution. The real threat to the life of the nation, in the sense of a people living in accordance with its traditional laws and political values, comes not from terrorism but from laws such as these. That is the true measure of what terrorism may achieve. It is for Parliament to decide whether to give the terrorists such a victory.

Lady Hale:

It is not for the executive to decide who should be locked up for any length of time, let alone indefinitely. Only the courts can do that and, except as a preliminary step before trial, only after the grounds for detaining someone have been proved.

Executive detention is the antithesis of the right to liberty and security of person. Yet that is what the 2001 act allows.

We have always taken it for granted that we cannot be locked up in this country without trial or explanation.

If the situation really is so serious and the threat so severe that people may be detained indefinitely without trial, what possible legitimate aim could be served by only having power to lock up some of the people who present that threat?