That would definitely not be a surprise.

Or is it because a lot of Brits are in hell so that's what he's used to?
Well, according the Brits, hell is full of the French
Rowan Atkinson (Mr.Bean/Black Adder) as the Devil:

because David Bowie was British. it's that simple.
Sean Bean has not died from Lightsaber related issues yetjust saying

"they" didn't tone it down. Tom chose to use an RP-like accent that he created for the role. He patterned the character after how he imagined a love child between Noel Coward and Mick Jagger would act and sound. That explains why he sounds refined with a touch of Mockney. Also why he comes across as a throwback and just a bit of a dandy. I think his portrayal is genius.
He's Welsh, but according to remarks made by his recent interviewer (posted in interviews thread) he didn't actually grow up in Wales. He clearly doesn't know Welsh, he demonstrated that pretty well. So I'm not sure what his own natural accent is, but it's definitely not the RP Lucifer uses.
"I have already lost touch with a couple of people I used to be" Joan Didion

Well, Tom Ellis is already British, so why not use that? Would be a waste otherwise. Btw, I was a bit surprised that his real life accent is actually much thicker: https://www.youtube.com/watch?v=S3sxJCGUIXk I guess they toned it down somewhat/made it more neutral in the series.

Because the Devil is British I should know he drinks in my local

No. Showrunner said Mom is unique. The angels are in their original forms.
"I have already lost touch with a couple of people I used to be" Joan Didion

Isn't it simply because when they move from 'the there side's (heaven or hell) they take the body of a random recently deceased person? His mother us shown taking a few after they all humourously get killed again. So just by chance Lucifer got the body of an English guy who died in LA.
It's the same reason that his brother got a black body and he got a white one. Pure chance.

Rat you're too analytically minded. Brits are well liked by female American audience members, and Tom makes the character appear a little more refined. Viewer manipulation increases ratings.

Where have you been the entire life of Cinema & Television?
I don't watch much TV. That must be the issue.

The rest of the family have American accents. So why doesn't Lucifer?
Where have you been the entire life of Cinema & Television?
The bad guys
ALWAYS
has a British accent, unless of course it's German.

I am from UK and have been to hell and back and its full of people from the UK (Mostly London) so the accent fits quite well!
Its uncle Frank Kirsty!
Its time to play!
Soon, we will all have special names!

Tom Ellis said in an interview that they tried the character with an american accent but he sounded too dickish. As a joke, he said that you can say whatever you want with a british accent and still sound charming.

I think it's because certain British accents have a natural authority and charm to them (depending on who is speaking of course).
And since Hollywood has often cast great British actors as villains since the invention of celluloid, using a voice that would normally be associated with a "bad guy" would make sense. Lucifer is meant to be the ultimate villain (at least that is the general perception), and therefore using someone with a British accent which many people associate with the "bad guy" makes it easier to accept that he is Lucifer..possibly?
Also irony and sarcasm somehow seem a little softer and easier to take from someone whose accent makes them sound like they are normally patronising or condescending to everyone anyway.
Please allow me to introduce myself. I'm a man of wealth and taste

English accent sounds a lot cooler in divine or mythical characters. Gandalf would sound like Balthazar from The Sorcerer's Apprentice in an American accent. But I get your point, at times that annoyed me too. Since they are butchering the source material, I just watch it as a CSI show, with some celestial detectives.
How the name, "Martha" brought out the real Batman
http://i.makeagif.com/media/8-18-2016/8C46Hb.gif

Hey, I like women with British accents. I think it's sexy as hell.

Heh one of my guilty pleasures is watching big bang theory just for Sheldon's accent. Love it lol
Gotta run.

Texas is referred to as a "Texas Twang".
Not sure about Louisiana, they have a classic southern drawl but there is also Cajun which is completely something else and is French based. I have a cousin that married a gal from the Louisiana Bayou country that spoke only French until she stated school. Not that uncommon there.

I love all of them really lol, but my favourite is the southern accents like Louisiana and Texan accents. Lovely relaxed drawl.
Gotta run.

Which American accents? There are so many!

Lol I know, only been to the US once and I felt like a celebrity. Couldn't talk without someone gushing lol. I think its reciprocal though, I love American accents.
Gotta run.

Well as it happens I am not American at all.
Don't be negative and assume someone is "offended" just for asking a question.
Perhaps it is you who are offended, just a thought.
Also, all of us here are saying British accents are hot. That's where my question comes from.

Centrd I don't mean Americans don't love British accents I mean every time there's a Brit or Aussie in an American Tv/film role some one ALWAYS pipes up and says 'why does this character have a non American accent' like it's offensive to them or something. I mean we're talking one character and the rest are all American.
Learning accents takes practice and some times a professional.

As a Southerner, I can confirm that non-Southern women seem to like the Southern drawl, at least to my experience. Though, as you said, they like the Southern gent accent, more subtle and slow, rather than the outright redneck twang that many have. Thankfully mine is the former.
I thought I was wrong once. It turned out I was mistaken