The lessons learned at the Falklands by the British government was the apparent utility of further reductions, eliminating first the SHAR and then the Invincible class carriers, along with massive reductions in numbers of destroyers and frigates.
Capability changes. The size of fleet isn't an issue if the ships are more capable and more deadly than the previous generation. The problem Britain faces is changing geopolitical situations and attempting to maintain a global presence. Britain's star has been on the wane since the end of WW2 and it is no longer the powerhouse it used to be, so politicians have had to adjust with the times in determining Britain's continuing role on the world stage. I'm certainly not defending them for getting rid of the Sea Harriers or the GR.9s - a huge waste of still competent aircraft being retired prematurely, but the biggest sin they have made is not specifically retiring ships and aircraft and shrinking the size of fleets, but retiring a capability without a suitable replacement. Britain can't afford a big navy or a big air force, it also arguably doesn't need one. Britain needs to consolidate its forces and review what its global obligations are. Britain is still a nuclear power, which brings with it its own security, but these things are costly, so what stays and what goes when defence budgets fluctuate with political parties?