North Korea has caused worldwide concern with its test of a new type of ballistic missile.
It could give Pyongyang greater capability to launch nuclear weapons from submarines.
Experts say it appears to be a reversion of a land-based projectile, with advanced guidance capabilities.
The United Nations Security Council called an emergency meeting, while the test has prompted condemnation from the United States and North Korea's neighbours.
Former UK National Security Advisor Lord Sedwill told Forces News it is a genuine worry for Britain.
"If people sometimes think that North Korea is a long way away and why is this a matter for the UK, it's worth remembering the globe is round," he said.
"Actually, Berlin is closer to North Korea than Seattle, London is closer than Los Angeles."
Despite the missile being smaller, with a shorter range than other similar weapons tested by Pyongyang, it could allow the state to carry more of them to anywhere they can take their submarines.
North Korea's recent run of tests might have something to do with activity south of the border.
Seoul has tested a submarine-launched ballistic missile. South Korea is not banned from developing the technology, but it does not have nuclear weapons.
Both states say they need the weapons to protect themselves from the other.