But did Johnny Cash ever actually spend any time in the clink?

Mick'n'Keef had their celebrated single night, was it, in Lewes prison, as the butterfly was broken on the wheel.

James Brown and Arthur Lee were both incarcerated for possessing firearms illegally. Arthur Lee did 12 years.

Do time for The Man?

Did he do any time in Japan, or was he just held in custody? And Springer is right, Crosby was sentenced after being stopped in his car in possession of a handgun while on parole following a drugs conviction.
Roy Harper has had a spell in chokey too.
Steve Earle did time for drugs possession while Merle Haggard witnessed Johnny Cash at either San Quentin or Folsom Prison from the wrong side of the stage. Apparently it inspired him to take up music as a career.

Layla's drummer is doing porridge for life for topping his mother. Agreed a bit more serious than our Pete's misdemeanors.

And of course Gary Glitter and J. King were E Wings' bitches for a few years.

Leadbelly

Jim Gordon (see above)

Hugh Whitaker (drummer of The Housemartins)

Housemartin Hugh Whitaker was convicted of assault with an axe, not murder. But he did do time.

Assault?

Knew about Chuck's various problems. Looked him up in Rolling Stone and found this wonderful phrase about the 1959 case under the Mann Act involving Janice Escalante, a fourteen-year-old Apache prostitute.

Escalante testified that Chuck "had been intimate" with her "in each of four states." Judge Roy W. Harper sentenced him to three years in prison and a $5,000 fine.

Steve Earle spent time in jail for drugs and firearms charges.
I like this, from the sleeve notes to his 1996 album "I Feel Alright", under the heading "Perspective":

When I was locked up, I was getting ready to go off on this boy that stole my radio. My partner Paul asked me where I was going. I said, "To get my radio - and then go to the hole for a little while." He looked at me like I look at my 13 year old sometimes and said, "No, you ain't. You're gonna sit your little white ass down and do your little time and then you're gonna get out of here and make me a nice record". So, I made two.

[The two records were "Train a Comin'" and "I Feel Alright".]

Much as this is an interesting list of those incarcerated overseas, by and large, there are precious few references to the question asked, those locked up at her majesties pleasure, = her majesties prisons (HMP), i.e. this country.
(But maybe this is what the questioneer meant, so what do I know?)

(Crossthread alert)

When Mr Doherty gets out he's going to be "ex-jailbird" for the rest of his career

got banged up at an HMP a few years back, didn't he?

Serves 'em right for stealing their name.

...as given out by the Grim Reaper.

a jail break somewhere in town tonight" As we all know the prison may be the best place to start.

Also Doherty has already done some porridge so he should really be referred to as "the Recidivist (ex?) junky model-botherer and general nuisance Pete Doherty"

Did time in the late 50's early 60's and saw Johnny Cash at one of his prison gigs and then took the Straight and narrow path or "Walked the Line" if you will.
Johnny never did"Bird" and the song were he was arrested for picking flowers was "Starkville City Jail".
Rapper Keith Murray has only recently been released fron Prison and therew as a Rap group caled Lifers Group, A group of inmates who Rapped.

Wasn't there a story about a terrified 'God' being banged up after a late night session at Gram Parson's place? Or the other way round? And of course Rocking Ronnie Wood and Keef spent a quiet night in after being pulled over for driving too slowly somewhere in the midwest. I read that in his book, a lovely prize from those nice folks at The Word. I'm still waiting for my t-shirt by the way Fraser....

Not a musician I know but I thought I'de bring it up as reading about him affected me quite a bit.
I've never met the bloke and I don't know the finer details of the case but I can't help feeling sympathy. He's obviously not well but he can get better, unfortunatly by then his career will be in tatters and the nation will have been robbed of one of its finest radio DJs, simply because he's human with human frailties. Surely if he'd got help and not been sent straight to court then he would still have a career he could go back to and that would be something he could focus on.
But like I said I don't know all the details so maybe I'm speaking out of ignorance.