Prisons change use over time as you said, and that's it - Strangeways, as it used to be called, wasn't men only then. It is now - prisons get re-roled according to need. For instance, I've been in HMP Wellingborough (now closed) and that was originally built as a borstal for teenagers. So in a way you've answered your own question. British prisons are all same-sex now but that didn't use to be true when there were fewer of them.
Plus, Categories didn't exist then. That's a 1960s idea. The prison review suggested that there are few enough Category A prisoners to put them all in one really secure prison. Maybe on the Isle of Wight so escapees would have the added problem of getting off the island - same idea as putting Alcatraz on an island so it would be hard to get away from. The government read the report and said absolutely not - that one prison will be the focus of all serious escape attempts so it's a daft idea, Let's split them up, so we have about 8 Cat A prisons. Which mostly serve other roles as well, like HMP Manchester.
That has other uses. If a prisoner and his friends cause trouble, you can split them up and move them to other prisons. You can't do that if there's only one Cat A prison. That was one thing I noticed - troublemakers got moved! I got moved twice but that was only because Wormwood Scrubs is a Cat B "local" prison so as a Cat C prisoner I didn't belong there, and again because Wellingborough didn't have the offending behaviour course I should be on so I got moved to another Cat C that did.
Dead right, hard labour was a thing when the whole idea was prisons should be purely punishment. Sometimes it was even said in the sentence - look at older laws and "with hard labour" is often mentioned. Breaking stones or picking oakum would be doing useful work, but walking on a treadmill that didn't power anything really was nothing but punishment.
Of course now we think about rehabilitation as well and work in prisons is to earn some money, so you WANT a job. I was a teaching assistant for a while and I liked that - if I can get someone who failed GCSE maths to finally get their head round some part of it, YAY! I did have someone who never seemed to remember what we did about equations and graphs from one lesson to the next, that's exasperating, but what can you do except explain it yet again and hope it sinks in this time? Makes prison education cheap too when I only got £10 a week for it!
Having done it, I can understand why teachers love what they do. Mr Hagan at Dormers Wells High School who taught me maths once wrote a school report on me that said, and I quote it in full, "If all my pupils were like Clive, I'd work for no pay!" No doubt there were others who didn't get it and that frustrated him, but what a fantastic thing to say. Did I make it feel like coming to work each day was worth doing? I hope so. Which reminds me of Mr Williams the head of history. In the sixth form he begged me to do A level history (it's one of my six grade As at O level) but no, I'm a science guy, I'm doing pure maths, physics and chemistry and I can't fit that in too!