The Merlins were being manufactured in England and at the time, there was a real concern wether England might be next to fall under Germany's onslaught.
Nice to know that you consider the rest of the United Kingdom of Great Britain and Northern Ireland was safe from Hitler’s predations!
The first German air raid on Britain on 16 Oct 1939 targeted RN ships in the Firth of Forth. Spitfires from 602 (City of Glasgow) and 603 (City of Edinburgh squadrons, based at Drem & Turnhouse respectively, were scrambled and succeeded in bringing down 2 German aircraft.
Rolls Royce had a shadow factory at Hillington, on the outskirts of Glasgow (in Scotland) from 1937, it completed its first Merlin engine about two weeks before the outbreak of WW2. It went on to produce about 14% of all Merlins produced worldwide as well as Griffons.