This is a tour de force but dare I say it the UK had no devolved governments before 1997, so the first-level subdivisions in 1939 were the counties, just like in Ireland. The Scottish counties are a bit of a nightmare though, I wouldn't blame you for glossing over them, but the English, Welsh and Nornirish ones are straightforward enough if you've got the stomach for them!
Nominated
At least I doubled my knowledge of French regions (2 to 4) and know about all of Poland. A lot of hours playing HOI4 also count a lot...
-Kieleckie should be accepted as Kielce
-Lubelskie should be accepted as Lublin
-Warszawskie should be accepted as Warsaw (Voivoidship)
-Nowogrodzkie should be accepted as Nowogródek
-Poleskie should be accepted as Polesia
-Wilenskie should be accepted as Wilno
-Pomorskie should be accepted as Pomerania
-Slaskie should be accepted as Silesia