Non-Proliferation Commitment: The NPT aims to prevent the spread of nuclear weapons by encouraging non-nuclear-weapon states to not develop or acquire nuclear weapons. The UK, as a responsible nuclear power, made a commitment to contribute to global stability by not assisting other nations in developing nuclear weapons or giving them access to nuclear materials.
Nuclear Disarmament: Article VI of the NPT calls for nuclear-weapon states to pursue nuclear disarmament in good faith. By signing the treaty, the UK acknowledged the importance of reducing and eventually eliminating nuclear weapons, while maintaining a minimum level of deterrence as long as nuclear weapons exist in the world.
Verification: The NPT includes provisions for inspections by the International Atomic Energy Agency (IAEA) to verify compliance with the treaty. The UK agreed to allow inspections of its nuclear facilities to ensure that its nuclear program is for peaceful purposes and not for developing or acquiring nuclear weapons.
Denuclearization of the World: The NPT is seen as an important step towards denuclearization of the world. By participating in the treaty, the UK showed its willingness to work towards a safer world, with reduced reliance on nuclear weapons.
Political Considerations: As a leading member of the international community and a permanent member of the United Nations Security Council, the UK had a vested interest in promoting a stable international order and preventing the spread of nuclear weapons. Signing the NPT was a step in line with the UK's foreign policy objectives.
It's worth noting that the UK is one of the few nuclear powers that have ratified the treaty as a non-nuclear weapon state. This means that the UK voluntarily gave up its nuclear weapons program in 1950 and has consistently supported non-proliferation efforts.