They had a tidbit about this topic on their podcast.
I get that and by the 3rd episode, it seemed more normal, but it threw me off at first. The opening scene with the engineers in the control room had me confused, are they in Russia, is this a nuclear scare that happened in England, it was odd. I'm only 3 episodes in and I've adjusted more or less, until someone comes on screen with a crazy heavy Britt accent, like the coal miner foreman, it was like, woah, am I suddenly watching an episode of "Peaky Blinders"? They could have at least some of the actors tone down their accents a bit.
It's kinda like watching a Civil War movie set in the Deep South, but everybody has German accents. It would be weird to have General Robert E Lee sound more like Field Marshal Rommel.