Trident replacement still under consideration
How can the UK claim to be committed to a world without nuclear weapons at the same time as renewing Trident? In 2007, Parliament took the decision to begin the design work required to make possible a replacement for our ballistic missile submarine fleet to enable the UK to maintain a minimum nuclear deterrent beyond the life expectancy of the current submarines.
This decision is entirely consistent with our international obligations (the NPT does not prohibit recognised nuclear weapons states from maintaining and updating currently held systems) and our commitment to nuclear disarmament. It will not lead to an increase in the size of our arsenal, nor does it commit us irrevocably to possess nuclear weapons.
The Prime Minister has made it clear that as soon as it becomes useful to do so, the UK stands ready to include our nuclear arsenal in a future multilateral disarmament negotiation.
http://www.fco.gov.uk/en/global-issues/weapons/nuclear-weapons/nuclear-questions