Some of the best words that I've read on this, as I say, come from the House of Lords decision. Lord Brown said:
Torture is an unqualified evil. It can never be justified. Rather it must always be punished.... But torture may on occasion yield up information capable of saving lives, perhaps many lives, and the question then inescapably arises: what use can be made of this information?
The court concludes you can never use it in criminal proceedings, you can never use it in judicial proceedings. But Lord Brown goes on to say:
Generally speaking it is accepted that the executive may make use of all information it acquires: both coerced statements and whatever fruits they are found to bear. Not merely, indeed, is the executive entitled to make use of this information; to my mind it is bound to do so. It has a prime responsibility to safeguard the security of the state