But Barrowman partially grew up in the US, he moved there aged 9 and stayed until at least college age. His accent is flawless because it’s his actual accent.
Linus Roache’s accent in Law and Order is a bit ropey but adequate. I think it’s still easy to tell he’s British and not American though.
As I said. 
Also, in one of the two episodes of Torchwood I saw, he say “Estrogen” all weird, like it had an extra E at the beginning.
No, no – he said “Oestrogen”. With the extra O.
I’ve seen him mentioned before. Does he really not sound American, or is it just you being hypertuned to what you expect to be a foreign accent? I mean, he is American. I know you said ‘As I said,’ but an Gadai was saying the opposite to you, so I don’t really understand what you meant.
IANARaguleader, but he does say that actor’s accent is “flawless” (An Gadaí explains why) - it sounds to me like what pings him as “strange” is, exactly, that flawlessness. IOW, he is expecting a flawed accent but the flaws aren’t there. It’s like the people who, either knowing I’m from Spain or having seen my name, have filed me under “Hispanic” before we actually meet and get confused when my accent doesn’t match their mental image of a Hispanic accent. Reality has this strange custom of not giving a shit about people’s preconceptions.
(I do have a Spanish accent, which gets stronger if I’m tired or haven’t spoken English in a while, but I’m from Northern Spain - actresses from Northern Spain used to get roles in the US playing Italians, for a while. We don’t sound like Antonio Banderas, we don’t sound like Cantinflas)
Wow, I think that’s the first time I’ve been the subject of an IANA disclaimer. 
What I meant was, if I am aware that the actor in question isn’t from the US, I will be listening for anything in the accent that doesn’t sound “right” to me. They might be putting out a completely flawless North Carolinan accent for all I know, but since I’ve never been to North Carolina, anything different about that accent from accents I am more familiar with will ping my “Fake accent!” detector. If I had no reason to believe that the guy was from outside the US, I wouldn’t be listening for any weirdnesses and I wouldn’t notice.
Really, the Estrogen line is the only thing I can think of that ever sounded weird with Barrowman, so I figured he had a really good American accent but a word or two slipped through due to some particular dialoect-based pronunciation quirk of the word. I mostly just picked him as an example for fun.
Now, one Brit who has a really good American accent, even when I know he isn’t American: Christian Bale.
Ryan Kwanten, who plays Jason on True Blood, is an Australian actor who does a very good American southern accent. Coming from the South myself (actually from Shreveport, La, which is a frequent setting of the show), I can usually pick up on fake southern accents immediately (as I think most southerners can), but I never twigged that Kwanten was faking it for the first couple of seasons, and was surprised when I did learn he was Australian. Even most non-southern Americans don’t fake a southern accent very convincingly.
Heath Ledger is another Aussie who could do very convincing regional American accents.
I hadn’t the slightest inkling that Jason was an Aussie. Colour me surprised and impressed.
I thought Kelly Macdonald in No County For Old Men did a fantastic job with a very specific accent.
And I remember some Brits commenting on how Michael and Christopher in Spinal Tap did an amazing job with a very specific London accent. Don’t recall exactly but it may have been east-end.
For what it’s worth, the accent always struck me as fine; it just always seemed like he was doing a half-assed Sam Waterston impression, which cracked me up.
Just needed a place to state how amazed I am at how Alan Cumming’s Scottish accent totally disappears while he plays Eli Gold on The Good Wife.
He did a good Minnesota accent in Sweet Land, too (at least to my ears).
I’m popping in to say that I have yet to hear a good Northern Irish accent done by an American.
I don’t mean “He’s supposed to be from EAST Belfast, not WEST Belfast” bad, I mean “Has this person ever heard an actual person from Northern Ireland speak?” bad.
Brad Pitt (what actually managed an OK Traveller accent in Snatch) was woeful as an IRA terrorist from Belfast in The Devil’s Own.
Tommy Lee Jones, Mickey Rourke, Richard Gere all being notably bad as well.
I think something that people forget about British and Irish actors is that the big drama schools (RADA etc) in the UK, which still produce a lot of the big TV and film actors, have classes by very good dialect and accent coaches- an experience American actors may not have had.
I grew up where Eternal Sunshine of the Spotless Mind took place. I can tell you that her accent was dead-on perfect.
This is one I hear a lot but IIRC his TDO accent was at the very least passable. I’ll have to check a youtube clip to see if my memory is deceiving me. His accent was definitely Norn Ironish and not oi tee toi tee toi Oirish anyway. 
I can think of two examples of excellent English accents from American actors.
Dustin Hoffman does a wonderful, over-the-top accent in Hook playing the evil pirate himself.
But the best one has to be from Angelina Jolie in the Tomb Raider films. Her accent in the role of Lady Croft is perfect as being that of a member of the British aristocracy. She is a countess after all (Croft that is, not Jolie). She had to learn to speak with an English accent specifically for that film.
Renee Zellweger’s accent for Bridget Jones wasn’t perfect, but seemed pretty good from my American point of view.
I think the main reason Brits and Aussies are better at it is because all they have to do is lose the accent completely and, BAM! They’re speaking American. Am I right or am I right? I’m right, right?
IIRC, she got into character by having a job-placement company put her “undercover” for weeks in a London publishing company – where she only ever spoke with a British accent that locals apparently accepted without question.