From:
http://www.mod.uk/NR/rdonlyres/31D096E9-3F41-4633-BEA2-AE62CF97C3AE/0/annrptvol1_200708.pdf
(page 39)

"28. The UK’s Trident submarine force has
maintained a continuous and independent
nuclear deterrent capability at sea, in support
of NATO and as the ultimate guarantee of
our national security. In June 2007 HMS
Vengeance completed the 300th Continuous At
Sea Deterrence patrol. Since April 1969 there
has been a Royal Navy deterrent submarine
continuously on patrol somewhere in the world.
Following the House of Commons endorsement
in March 2007 of the Government’s proposals
for maintaining the UK’s nuclear deterrent
capability by procuring a new class of
submarines to replace the Vanguard-class
and joining a programme to extend the life of
the Trident II D5, work began on the detailed
concept phase for the new submarines and
on taking forward participation in the Trident
II D5 life extension programme. The MoD has
continued the programme of investment in
sustaining capabilities at the Atomic Weapons
Establishment, both to ensure the UK can
maintain the existing warhead for as long
as necessary and to enable us to develop a
replacement warhead if that is required. To
inform a decision on whether and how we may
need to refurbish or replace this warhead, which
are likely to be necessary in the next Parliament,
work was taken forward to review the optimum
life of the existing warhead and to identify the
range of replacement options that might be
available."