Take the RAF back to the 1940s, 50s and 60s, recruit lots of Poles and Czechs.
Don't make me say the 'B' word!
On the point of Commonwealth citizens serving with UK armed forces, I read a couple of years ago that South Africa was moving to prohibit its citizens from taking up arms for any foreign government or organisation, as the government sees them all as effectively being mercenaries. Not sure if anything came of it.
Here is a link
https://www.news24.com/SouthAfrica/N...robed-20030408