I read Dr Fox's comments viz MRA4 as positive. It appeared to me that he was effectively trying to ringfence those capabilities by pointing out that not only are they multi-role, multi-task platforms/units but also the danger of being unable to re-generate the capabilities if we decided we needed them in the future.
And that is exactly why he allowed it to be leaked. Why on earth would he write a letter to the PM on such a fundamental level? The answer is that he wants to be seen as a minister representing his department and one who cares for his troops. IMHO the PM approved the letter and knew that it would be leaked. Everything in that letter is a done deal and the PM is happy to be seen as the bad guy.