I think there's a fair chance that he did not expect a justification, though was making the point that it was not the PM who had to deliver the news face-to-face, i.e. it's much easier and not necessarily justified to make decisions that appear arbitrary, though not so easy to implement them for those who effectively do so. The over-arching point being the decision is, by implication, incorrect, which I would hasten to say is the case. I suppose it's really about, in the way he put it, the difference between politicians and others, including and perhaps more so, the military. This is as you stated about the difference between the perception of a two-way street and 'reality'- and moreover as the politicians shape and have further shaped it (to everyone's detriment, it would be fair to say).
It's interesting that instead of address efficiency in some of the largest areas, in fact the largest government employment area- the NHS, they left it unchanged in terms of the budget. As they did for other areas. This is a political gesture and one to safeguard their 'cutting' reputation as the NHS is a 'touch-rail' issue in the UK, as medicare has been in the US for decades. This I believe, with the effective substitution of capability for reform, be a more expensive (in the more substantive sense also) than addressing all that needs to be, and fundamentally so. This trend is also mirrored in their approach to education, which will be highly damaging into the future, and is, when one looks at the figures and other expenses, regressive as the IFS Institute of Fiscal Studies) has stated.
A.