RP,
£3Bn also represents approx. 6-7% of current interest payments on the UK national debt (approx. £40Bn) and by 2017, when we might be looking to get our first airframe if ordered in 2015, approx. 4% of interest payments (expected to be approx. £70Bn, or over twice the defence budget, by then).
As HAS59 said, there is money being spent all over the country, that's why we are still borrowing about £120Bn a year!!
Yes, we could spend £40-50Bn on defence, and could reinvest in MPA, but the politicians have set a smaller budget, which the military has to work within. If we want to get back into the MPA game (and I'm not saying we shouldn't), given the current budget, something else has to go? What?