The army is a very large and complicated animal and I don't know if the army is above or below strength at the moment.
Just counting the number of soldiers is I suspect a huge over simplication of the problem. You have to have the right number of soldiers at the right place in the right job. You have to have the correct proportion of the army in training, the correct proportion at all rank levels and the right amount ready to leave.
If we have all of a sudden managed to recruit a spare 5000 bodies and fill the gap, they'd all still be in training and operationally 'useless'. They might also have been recruited to fill the 'choice' trades leaving the infantry, armour and other arms still short of manpower. People may also be leaving quicker with the increase in Op tours and the improvement in the country's economy.
Currently the whole Army is about 3000 undermanned (depending on whether you count FTRS, etc), with the trained soldier inflow exceeding outflow for the last couple of years by well over a thousand a year, and so the gap is closing.
The problem that the Army currently has is that a combination of the recruiting surge going better than expected, and with voluntary outflow rates the lowest they have been since about 1996 (and still falling), we are in a position where we are 'not as undermanned as the bean counters thought we would be' therefore the MOD can't pay the wage bill! Hence the recent restriction on FTRS extensions, and allowing PVRs to leave early (both debated endlessly on other threads on this site). In fact some bits of the Army are overmanned - AGC (SPS) and Royal Armoured Corps being two examples. Check out the DM(A) webpages.