real growth, above inflation, RPI etc etc measures only make sense if they are comapred on an otherwise level playing field - in simple terms, if the UK was not involved overseas at all, and was merely running peacetime detachments to all those nice places (and some not so nice) that we traditionally pop across to, then a defence budget that keeps pace with inflation is fine and dandy.
On the other hand, if you take on extensive overseas commitments, it stands to reason that you need to inject a great deal more cash - it costs a great deal more to keep people in Afghanistan than it does to send them to the holiday inn, Jacksonville for 6 weeks a year.
The budget has to be related to the task, otherwise you spend all your money on the sharp end, the support branches are left to wither in the hope that the damage will not prove terminal by the time the sharp end business has been concluded. If the task runs into a multi year commitment then you need to budget for the higher level of expenditure - a few percent doesn't cut it. You simply can't go to war on the cheap. (Almost) unfortunately, since WWII our conflicts haven't been total war situations, which seems to have deluded some into assuming you can project power half a globe away for an extra fiver or two.