It’s been quite a while since the UK was in a sticky situation and on the receiving end of volunteers, but:

en.wikipedia.org/wiki/Eagle_Squadrons

The Eagle Squadrons were three fighter squadrons of the Royal Air Force (RAF) formed with volunteer pilots from the United States during the early days of World War II (1940), prior to the United States’ entry into the war in December 1941.

My assumption is that the author doesn’t object to this.