The problem is that we need quite a lot of most things, and the training to use them well. Some surprising organisations have had to re-learn what they'd forgotten - like desert ops, which Hereford had forgotten before GW1, hence putting in foot patrols into the Iraqi desert, which was a disaster.
Setting up three or four squadrons just for COIN would take at least six years - maybe more.
But to be training for such a huge range of operational tasks, in all the different terrains takes lots of manpower - you don't turn mountain troops into jungle troops in a long weekend, aside from the variations in kit required.
If HMG wants the Services to be a global police force in addition to protecting our borders, interests and trade routes then they need to be prepared to pay for it, and right now that seems to be the last thing any politician is prepared to have an open discussion about.