NutLoose,
It certainly is, for most senior appointments, heading your respective service is usually the retirement one. What's interesting about this, is since the first full time appointed head of HM Forces, Sir William Dickson (incidentally had an uncanny resemblance to Sir Stuart Peach), in 1956 this is only the third time that a former Vice Chief of Defence rather than an individual service chief of Staff has been appointed, the out going Nicholas Houghton and Lord Vincent in 1991 being the only other two. Also interesting that a senior officer who has never held either such prior appointment was also considered and the preferred choice of the MOD.
Personally I'd have thought Sir George Zambellas would have been appointed CDS, the last Naval chief was Lord Boyce; 2001-2003!
FB