It seems to be Urban’s natural NZ accent with a light layering of cockney over it. I’m technically a cockney and grew up around the accent, so I kind of assumed he was meant to be a New Zealander or Australian who had lived in London for a long time or something, but later in the show he’s meant to be totally a Londoner. It’s odd, because Urban never seemed to have problems before with accents - his Bones was pretty great for example. So I have to think it’s some kind of wacky artistic choice, but it’s a bit weird. Maybe next season they can retcon him to being originally from NZ or something.
Re: the over-the-top elements, I think it’s because it’s Garth Ennis and from a very certain era. Ennis’ depiction of America and particularly frequently idolized or fetishized elements of it is heavy-handed, but also a reaction to how uncritically a lot of that stuff was regarded well into the 1990s, and indeed, in some parts of American culture, still is. It does get a bit excessive but it also does at least feel like an honest reaction, to me, to sheer density of hypocrisy and corruption in a lot of his subjects. And if power corrupts, and it clearly does, as much now as ever, then superheroes who aren’t corrupt is a kind of ludicrous fantasy.
So yeah it gets a bit OTT but at the same time I kind of like it and feel like it still has relevance, sadly.
I feel like the “power corrupts” angle is why so many superheroes sort of end up replaying the first few years of their career over and over. If one was even slightly realistic, after a couple of decades (or much less, in some cases), a huge number of people who started off idealistic and heroic would have become monsters. And the relationship between power and corruption is something a lot of media is still very reticent about acknowledging, because, I guess, it’s more fun to have your cake and eat it, as it were. Also, even without corruption, someone who was a champion of mainstream values in the 1930s, or 1950s, or maybe even 1980s might well be a monster by the 2020s, if they didn’t change with the times, but I guess getting off-topic for The Boys there.