You keep telling yourself that renfrew.
You might be surprised how quickly free prescriptions, no university fees, freezing council tax, real terms increases in health services and benefits associated with an unemployment rate that is currently the same as that for the whole of the UK all eat into your GDP.
You might be able to afford to maintain a few infantry battalions and a couple of light aircraft at the start, but do you really think you could sustain any credible capability over the long term - operating costs, programme sustainment, personnel costs and recruitment and capability replacements?
If you aren't part of NATO, you won't be able to claim for assistance there and I wouldn't bank on walking out of the UK and straight into the EU for assistance there either.