The third, the Vigilant, was launched in October 1995 and entered service in the fall of 1998. Technically, there are no specifically American or British Trident IIs. A pool of SLBMs are kept at the Strategic Weapons Facility Atlantic at the Kings Bay Submarine Base in Georgia. Britain has title to 58, but does not actually own them. A missile that is deployed on a U.S. SSBN may at a later date deploy on a British boat, or vice versa.
Several factors go into the calculation.
We assume that Britain will produce enough warheads for only three boatloads of missiles, a practice it followed with Polaris. But the purchase of only 58 missiles means there will not be a full complement of missiles for all four boats. The government also stated that normally only one SSBN will be on patrol, with the other three in various states of readiness. Some Trident II SLBMs have a single warhead and are assigned targets once covered by WE177 gravity bombs. There is also some flexibility in the choice of yield of the Trident warhead.