I'll not be arguing that this show not be classified as United Kingdom, English, just point out that such classifications generally reflect the origins of the production, not the location of the shooting or the location purported to be depicted in the show. Although screened by a UK company the production was apparently commissioned by A&E Networks, which according to Wikipedia is ".... an American broadcasting company that is a joint venture between Hearst Communications and Disney Media Networks, a business segment of The Walt Disney Company." This is reflected by IMDB citing Country as USA.
I'd also be pointing to the sprinkling of Irish accents amongst minor characters in the show, as reflecting not just the location of the shooting but also the curious affection that the USA has for anything remotely Irish. As for location I'm not really convinced that Dublin looks anything like London, but that's just a personal opinion.