https://www.gov.uk/government/public...ng-budget-2017
Charts 1 & 2 in the link are a good starting point. FY18 budget assumes a ~ £58Bn deficit between outgoings (spend) and receipts (taxes etc). Debt interest alone is almost the size of the defence budget, which is the fourth largest single departmental budget. Trouble is, the three above it comprise 60% of the total and are largely ring-fenced, one reason the pensions triple lock is under scrutiny. Add in debt interest and social care (from impact on NHS etc) as largely inviolable and you're at nigh-on 70% of the spend budget is committed.
Which leaves the rest of the departments fighting tooth and nail to retain their "share" or accept cuts / "efficiency savings" which can have a disproportionate effect on "output" - often expressed in reduction in manpower and subsequent retention issues.
Or we find a way to up the tax take by about 8% to remove the deficit and eventually release more money through reduced interest payments.
In reality, it's probably somewhere between the two, because it's difficult to see where a huge extra dollop of taxation can land easily and because at some stage we're going to have to stop routinely living beyond our means - particularly wrt funding overseas development, unlimited NHS care, triple-lock pensions etc.
In that context, extra money for defence is in the noise. What would be nice is a realisation that 2% GDP is a minimum, not a target to be feted. Just going back to 2.5% GDP would put an extra £12Bn a year in the pot.