Watch enough gladiator movies (stop that!) and you realize Ceasar, Brutus and their cronies, Italians all, speak with British accents.
Why?
Thomt
Not just Italians; in U.S.-made movies, just about any European has a British accent.
I think the answer is a two parter: Because so many Europeans do speak English with a British accent (British English teachers are a lot closer than American English teachers), but probably more so to make them sound exotic but still speaking English in a regal way. They would have spoken Latin, or some characters would have spoken vulgar Latin which became Italian. But it might sound silly for Caesar to speak with an Italian accent; he’d end up sounding like one of the Marx brothers or a gangster. For some reason I’ve never figured out, Americans project a degree of nobility on people with a neutral British accent. I guess we think they all sound like the queen or something.
Suspension of disbelief.
Right or wrong, we tebd to think of America as a “new” country. Americans didn’t really exist more than three hundred years ago; therefore, something set over three hundred years ago can’t have American accents without sounding anachronistic and incogruous.
So what accents should ancient people use? You can’t have them speak with “foreign” accents, like Italians speaking English, because they were speaking their native tongues. And you certainly can’t use subtitles. The only real choice is a British accent. It isn’t “modern,” because for all effects and purposes, the British have been around “forever,” or at least since the Middle Ages, which is long enough. So long as it isn’t an accent we attribute to a single time or place, like a Cockney or Manchester accent, British speech is temporally neutral
And besides, Shakespeare wrote plays about ancient Romans, and Shakespearean actors speak with British accents. I guess we’re just used to it.
I have heard it said that ‘Romans’ are often depicted with English accents because the Romans are billed as the authority figures and Americans associate English accents with authority - I guess it’s a British Empire-Roman Empire thing, and putting on Italian accents would just sound stupid, added to the fact that we don’t even know if Romans actually had Italian-sounding accents.
However, I would also like to know why villains are often depicted with English accents.
Italians speaking English don’t sound like they have a British accent to me - they sound like Italians speaking english with an italian accent! Maybe it’s all just a matter of perpective. I remember a discussion with an American who could not believe she had any trace of an accent whatsoever ?!
BTW, obviously, there’s no such thing as a ‘British’ accent.
Welease Woger!
Nonsense - there are plenty of British accents. 
I would just love to see a film of Caeser speaking in a broad Yorkshire accent
Or tea with a Geordie speaking Mussolini
I think it’s probably something to do with the Shakespearian element. Not that Shakespeare necessarily spoke like a stereotypical modern Shakespearian actor*, or even intended his plays to be voiced that way…
*(think Patrick Stewart for a not-too-pretentious example, or Brian Blessed for a rather-more-pretentious one)
There is no such thing as a single British accent in the same way there is no single American accent. But American newscasters across the country are all trained to speak with a “neutral” accent (Dan Rather sounds different in his broadcasts then in a casual interview where his Texas twang comes through), which some people describe as Midwestern. Same in England (as distinct from all of Britain, although Americans often say British when they mean English). There is a neutral English accent you’ll hear on newscasts which is the de facto standard.
The ones you’ve noticed – they weren’t all playing the Devil incarnate, by any chance?
There was a conscious decision, I understand, in the movieSpartacus to portray the Romans with British accents and the slaves without (mostly with American accents). The sole exception was Jean Simmons, who was explained as having the tony accent because she was a tutor. (although that doesn’t explain why Tony Curtis;’ character, also a tutor, has a Brooklyn accent so thick you could cut it with a knife).
I suspect they may have been inspired to do this in part because they had so many Americans playing slaves – Curtis and Douglas and all – and mostly British actors in the Roman parts – Charles Laughton and Laurence Olivier – and they didn’t want to have peopl put on fake accents. But it worked out well – it gave a sort of not-really-noticed class distinction between the two groups, and the British accent carried a suggestion of “Empire” that fit in well with the Roman Empire (although in this picture it was still the Republic)
That was my point. There’s certainly a neutral English accent (I have one!), but there certainly isn’t a neutral British accent.
In the HBO/BBC series Rome, the patricians and nobility speak with stereotypically upper-class English accents, while the plebeians and soldiers and so forth speak with stereotypically lower-class accents. And the slaves tend to speak with non-descript “foreign” accents to indicate that they come from some far-away tribe.