The Guardian has a nice article upon accents in films, and lists the top 10 worst accents. Surprisingly to me, Dick Van Dyke comes in at Number 3 (I have met Americans who honestly could not see what was wrong with the accent). Perhaps I shouldn't be too surprised as I haven't seen Keanu Reeves in 'Dracula' and can't recall Marlon Brando in 'Mutiny on the Bounty'.
Jane Leeves as Daphne in Frasier is right up there for me (I was living in Manchester when Frasier was first transmitted). I pointed this out online and an American said to me 'Show's what you know, she's actually British'.
I responded 'So am I'. They did keep pushing the point that as she was British could do a British accent, but did not comprehend that there was no such thing! The person also seemed to be suffering from that American delusion which is still present, but less so than it used to be, that everyone they speak to online is also a Yank.
Hints for the a-merkins... The UK may be a small country (group of countries!), but the range of accents is vast, the changes happen much faster than in the US. It would be just as difficult (perhaps more so) for me to do a convicing Brummie or Newcastle accent as it would to do a convincing Deep South accent. Though accents are becoming more homogenised with time, there remains much distinction.
This means that someone from one end of the country is not born with the ability to sound like they came from the other. Similarly, to UK ears some accents in the UK sound delightful, some harsh and some ugly (I won't name names, as personal tastes differ).
Personally, my accent is probably 'educated south east'. Not 'Posh' or RP (old style BBC), but the 'neutral' accent which so many people have these days as a result of moving around and mass broadcasting. I do have various anomolies as until about 10 or 11 I lived in Catford, London. Then moved to Kent, to Birmingham, to Manchester and back south to Surrey.
For example, I will sometimes slip into 'northern' pronounciations of 'grass' (hard 'a') and path. (The southern pronounciation slips an 'ah' sound in there, pahth).
As I was bought up in South East London I will slip into more 'esturial' phrasing at times (Sarf East, innit). I think this sounds ugly, and can bring myself up short if I catch myself doing it.
'Estuary English' is so named as it is an accent starting from the east end and spreading along the Thames Estuary into Essex and Kent. A slighly posher version is probably the nearest thing we now have to 'mainstream' BBC English!
Up until fairly recently, linguists claimed that they could tell from an accent not just the town of birth, but also the area and sometimes the street - but now people are moving around more.
Jane Leeves ultimately did improve on the Daphne accent. Either that, or we learned to ignore it.
She does a pretty convincing (to my ears) mid-west US accent though...! She was demonstrating this in a UK interview last year when she was talking about the US election.
Oh, two more hints for the Merkins. Australasia and the UK, very different accents, completely different hemispheres. In addition, there are other countries out there, most people online are not from the US of A!