How Politics Works
I can answer why cuts happen even though Defence spending goes up although the Minister probably wouldn't. It's simple really. On average the economy as a whole grows by something like 2-3% and the increase in the Defence budget usually reflects this to rise at about 2-3% with occasional exceptions. Defence costs on the other hand go up on average by 7%. This is because of pay rises (which we always like to be a bit higher than inflation and inflation is sometimes higher than economic growth) and the replacement of old equipment with new supposedly more capable equipment - the research and development costs for new equipment have to absorbed by the defence budget(and what can be exported).
So the budget goes up by say 3% and the government can say that it has made a modest increase in Defence spending but as our costs have gone up by 7% there is still a need to find about 4% of cuts - every single year. It does not of course take the brains of an Archbishop, or even a Prime Minister, to realise that this cannot go on for ever. But as we know for sure now, politics has nothing to do with resolving difficult problems faced by the nation and everything to do with protecting the lifestyle of its practitioners.