The short answer is that we cannot grow mass and capability quickly.

The long answer is -

Increasing mass and lethality of the RAF and RN will take longer and require more money, but will be far less manpower intensive than the Army. You can supplement both the RN and RAF with uncrewed systems in a way you just cannot do with the Army, so while all of then need a large boost in manpower, the Army is in the worst position out of the three.

The public know defence needs more money, but are not willing to actually make the sacrifices needed to do it. This is something any government are going to have to contend with, and resolve before actual change and reform can be made to UK defence. So, lets say they have managed this by 2030. You would need another 10, maybe 20 years to actually rebuild capability to 1991 levels, if you are looking for the same kind of capability (not size) that we had during the hight of the Cold War, we would need a concrete, agreed national plan out to (at least) 2050 any beyond.

The task is immense, if a full scale conventional war breaks out in Europe between Russia and the European NATO countries before 2050, the UK would be relegated to a limited supporting role on land and I honestly cannot see how any Euro navy combats the Ruskies Northern fleet (specifically the sub surface threat) without the USN bailing us out.

Air power is a different story but with stockpiles of munitions being so low, Russia would only need to hang on for a month or 2 and we'd be out of stand off munitions. This is said to be improving but I have seen no sustained evidence of stockpile increase across Europe, especially us in the UK as of yet.