Ah yes, I remember
The comedian Jimmy Carr shows how to do several English regional accents (and make everyone laugh!)
I listen to a lot of older BBC radio dramas, often originally broadcast from the 70s to the 90s. It’s interesting to hear how much the on-air accents have changed, from mostly London-based (RP, middle class posh, Cockney light) to much more regional.
Today I listened to one called The Blackpool Detective, and the locals had a very identifiable accent that I have heard a lot in the past but I didn’t know where it came from (and I can’t say if it is authentic Blackpool or not).
What if they’re doing Southern?
I sometimes find NZ accents harder to distinguish from some South African accents rather than from Ozzy ones. It can be noticeable during rugby union coverage at times.
You are choosing extremes to make your point. Glaswegian is a notoriously difficult accent. I have no problem with understanding even strong accents from any other part of the UK. And I identify them as being from the UK before I identify which region.
As to American accents, watch The Wire and tell me you can understand the Baltimore black guys’ accent easily - I’m usually pretty good with accents but I needed the subtitles on sometimes.
As to your last phrase - if you are referring to me, I’m Australian and lived in the UK for a few years and go back every few due to various UK connections. I’ve spent substantial time in Newcastle and understand Geordie. Strong Glaswegian is often beyond me.
But to be clear, if your point is that there are regional accents in the UK, then yes. Obviously. But the point SanVito made with which I agree is:
I used to work with an executive at my ad agency who was from New Zealand. In agency life, we often talk about “decks” (PowerPoint presentations), and when he said the word, he pronounced it “dick” (with a very clipped NZ pronunciation): “How’s the dick coming along?” It took a bit of getting used to. 
The “sex” vs “six” thing provides a basis for much Aust/NZ comedy
As in (N - entirely - SFW) Michelle Gomez on top form:
As a Brit, this is probably the one and only time where I struggled (initially) to understand any US accent. But I soon adjusted.
Funnily enough, I was recently in France and one point I could hear some people talking amongst themselves, and for a second or two I assumed they were French. But then my brain adjusted and I realised they were speaking English, but with strong Scottish (probably Glaswegian) accents. I think the tone/where the sound is formed in the mouth (more at the front than I’d be used to hearing in England??) of their accents was closer to French?
Ha, you obviously worked with my old creative director from Auckland!
What struck me on my trips to Australia, as opposed to my trips to Britain, was that the Australians used “thanks” in place of “please.”
At a bar: “What’ll you have, thanks?”
At a casino: “Place your bets, thanks.”
On a city bus: “Step to the rear, thanks.”
Maybe it’s not so much an accent as it is word usage. I never heard such things in the UK, though I’m sure they exist.
..and if they say it with substantially fewer, then they are English !!
True, though I’m not sure exactly what. As an American who’s listened to a lot of British English in my time, I still notice when I watch early episodes of QI that I don’t really distinguish the accents of Stephen Fry and Alan Davies. 
I can tell that their voices sound different, and they make different choices about vocabulary and sentence style, and I know that Fry is considered to talk a lot “posher” than Davies, but they both basically just sound “British” to me.
Must be like East Asian English learners having difficulty distinguishing semivowels “r” and “l”. It’s totally obvious to the native speakers who learn these phonetic distinctions from birth, but much less so to others.
There’s a notorious parody ad sequence about the NZ “deck” pronunciation:
About Australian accents, I’m surprised nobody seems to have mentioned the classic Let Stalk Strine by Afferbeck Lauder:
I was wondering if my wife posted this thread. She can’t recognize the difference between English, Scottish, Irish, Australian or Kiwi. Personally I can hear most of it but not the difference between some of the regional English accents. We were watching Dept Q and I paused it during a seen where Matthew Goode is talking to Kate Dickie. A very clear difference between Scottish and south England. My wife could not tell the difference.
When it comes to the Australian accent I think of John Noble. When acting he is often doing an English or American accent. To me they sound very similar because his American is very mid-Atlantic. His real speaking voice is very Australian.
On the other hand, I used to have a roommate from North Carolina, and when a British actor conflates “classic US Southern accent” with “Appalachian” or “Texas drawl”, it’s wincingly apparent to me.
Speech is weird.
BBC day player American accent is usually horrible. And you are right, regional is a lot harder it seems. The first time I saw the English actor Paterson Joseph I had no idea he was English. His American accent was that good. Then he was on Grantchester playing a MLK style preacher/civil rights leader. Everytime he opened his mouth it took me right out of the story. It was awful.
Bruce Boa made a decent living being the resident “American” used in a lot of British productions. He really wanted a Waldorf salad. (He was actually Canadian living in the UK)
I’d say British people would have a similar problem distinguishing Canadian accents from some US accents, particularly ones from northern states (the giveaway is indeed the stereotypical ‘about’, although it is nowhereas pronounced as, say, the South Park mockery suggests). So I do get why some Americans struggle with British vs Australian, to some degree.
Deep in Cajun country comes to mind.
If we’re going to talk about this topic, let me mention the currently showing film Wuthering Heights. It’s based on a famous novel written by an author who lived in Yorkshire. Apparently the novel and the movie are set in Yorkshire. The movie was filmed in Yorkshire. The two leads in the movie, Margot Robbie and Jacob Elordi, are both Australian. For those of you who’ve seen the current film, do they ever slip into Australian accents when talking to each other?