The United Kingdom is one of eight, probably nine nuclear powers. (Israel has never confirmed that it has tested/operates nuclear weapons.) Unlike most countries, the UK only uses one delivery system to operate its weapons: submarines using Trident ballistic missiles.
The British Trident system became an issue in the coalition government. While it was supported by both the previous Labour government and the Conservatives, the programme had to be reviewed per the coalition agreement. The review has been completed and it will be made public. But to explain to the public how Trident works, the BBC created this graphic. It does a really good job of showing the reach of the British submarines from one location, but then showing why an adequate replacement would need at least three to four submarines.
Credit for the Trident graphic goes to the BBC. The table is my own work.