Mildred
I don't work in a prison but I have been to lots of them, so can answer some of your questions until someone real comes along:
is the food completely terrible?
Varies - It often looks bad and smells terrible, and is served a weird times, eg. 11.30 am lunch, but then some prisons try to teach the prisoners catering and grow their own veg, and this can lead to quite good food in some cases and one or two prisons have naice restaurants that are open to the public.
do people constantly abuse each other or does it settle down and can it be an ok, friendly place to be?
Can be both - vulnerable and very nasty prisoners are separated for their own/others protection. A lot of prisoners just want to keep their head down, do their time and get out alive.
do people talk about their crime a lot, or just general day to day stuff?
Guessing - some do, some don't. They have psychologists and rehab and stuff. I think a lot of prisoners sometimes make good friends in there so will talk about the stuff that everyone else does.
is everyone utterly miserable all the time and dying to get out, or can they just sort of settle down and get used to it?
I think a bit of both - some will miss their families etc terribly, while others get institutionalised and just go in and out of prison - if they have been homeless or struggling to cope on the outside, it can be seen as 'better' to be inside, as you get fed and a bed etc.
do people do a lot of useful stuff with their time? Do you find that nearly everyone has a thing, like, they are gym bunnies or learning a language or something?
There are lots of classes and workshops - literacy/exams, learning trades, gardening, catering, libraries, manufacturing and yes, gyms. I've even heard of prisoners doing call centre work.
Are things very different in men's and women's prisons?
In some ways yes and no - there's still the risk of violence and bullying, but a lot more women suffer mental health probelms and selft harm/suicide. But the facilities are similar, although women's prisons usually have a mother and baby unit, but I don't know if there are ever father and baby units in mens prisons.
The Prison Service website has a bit of information about how prisons work etc
www.justice.gov.uk/offenders