Do you want the UK to reley on either the US nuclear forces or the French, Force du Frappe. Sorkosay has just said that France will retain its nuclear forces, full stop.
Independence is the only way to go, air launched or silo based systems are vulnerable to both, deployment difficulty in third party countries in the case of air launched systems and silo based is vulnerable as it lauch point is a known variable.
That potential adversaries know that an SSBN is a significant undectable? threat to their exisitance. It does not stop IED based devices smuggled into a country via a container. It's remarkably easy to loose a container and issue new paperwork it would appear. Remeber you only have load it on a ship or waggon truck around Europe for a few weeks and use a remote timer or command detonator. Look at the number of stolen cars moved using this system.
At the moment it is a percentage of other nuclear states that have a significant delivery capability, it does not mean that this will always be the same. Remeber in the early 80s, no one forsaw the fall of the Berlin Wall and its subsequent developments in Europe and the World.