I dont know if I ever posted this before but I find it quite interesting and useful.
The first is the Golden Jubilee edition (2002) and I have linked that editions Special Feature Strength of the Fleet page.
For 2006 I linked Chapter 3 - Formations, vessels, aircraft and vehicles of the Armed Forces.:
UKDS 2002 Golden Jubilee Special Feature - Strength of Fleet Frameset
UKDS 2006 - Chapter 3 - Formations, vessels, aircraft and vehicles of the Armed Forces
This is a great resource not only to track the Royal Navy on a yearly basis but also the other services.
Not to mention it also has data on the CFE treaty for all nations not just the UK.