With a floating Pound the problem of financing disappear as happened later in the Second World War. Note that they would be forced to tax heavily and emit bonds in order to retire money from the economy and avoid inflation, but it would not be a problem of 'where to get the money'.
In times of war, the veil disappear and it becomes clear that the important thing is how much the economy can really produce, not how much money the country has.
You don't necessarily have to tax. Saving acts like taxation, and a £10 note is just a permanent zero interest bearer bond.
Ultimately if you can't buy anything (rationing) or do anything (planning) then all you can do is pile up the money as savings.
Interesting how much harder it was for the government to get money back then.
Wikipedia seems to say that WW2 cost the equivalent of $4.5 trillion, while the Iraq war is about $2.4 trillion.
The Gold Standard seemed to have caused trouble with wars: WW2 had to temporarily suspend gold trading, and it was canceled entirely around Vietnam. Now we can spend trillions with it causing hardly any day-to-day trouble: A blessing when needed, and a curse when not.
Per this 2.2% of GDP in 2016 - after iraq with no 'hot' conflict. Google says 5.66% in iraq peak.
Percentages of estimated net world defense spending in 2016:
* US: ~36.2% of total world defense spending
* China: ~12.7%
* EU Countries listed: ~12.2%
* Russia: ~4.5%
I was talking about it from a inflationary perspective. QE didn't lead to inflation in the US example because of the way it was carried out.