Are we talking about the army or military as a whole? In terms of the army, a two-hundred thousand figure is itself fanciful due to a multitude of reasons.I think the figures you quote for a U.K. standing army of between 300-500k is a fantasy figure - only under full mobilisation (conscription) could we hope to achieve those levels.
I’d say 200K is the realistic figure we should be looking at. Even that figure would take years of reform and restructure to achieve.
We'd be decades off having that figures unless we saw a mass uptake in recruitment and spending, and even then we'd be massively short of equipment.
I mentioned a few days ago that the BA currently has the equipment to put a maximum if around 40,000 troops in the field, with the effective figure much less.
We've shrunk the number of permanent defence bases (e.g. beds); the stocks of personal equipment has been reduced; in terms of armour, k'in ell it's tragic.
Many regiments have been amalgamated and lost, with countless battalions disappearing as part of the efficiency reforms.
In short, the army is less than half what it was in '89 when you consider the number of regulars.