Do prison guards stand on their feet during entire shift (UK only please)?

They are not guards, they are officers.  You must have been taking notice of too many Americans.

They are standing the entire time they are not in the wing office and prisoners are out of cell, and there's a lot of walking around needed.  How else do you keep an eye on prisoners when they're out of cell?  On the other hand, prisoners are locked in their cells most of the time and are only let out for exercise, to come and go to work/education/offending behaviour courses, to collect meals and for association (which means time out on the wing to be with other prisoners, make phone calls, have a shower and ask the officers anything).  It depends on the individual prison how long any of these periods last.

If you've seen TV documentaries about prison, the one big thing that makes them unrealistic is they don't show the hours and hours when everyone's locked in.  Of course they don't show that because it would bore the viewers rigid.  Prison is basically very very boring.  I got through long Dickens novels from the library I'd never have thought of reading otherwise!

In my experience of Category C prisons, the longest period on weekdays with cells unlocked is evening association, which will be at most 2 hours after dinner until it's time for bang-up for the night.  But it will be a lot longer at weekends - then it's basically all-day association with bang-up for an hour at lunchtime and an hour at dinner to give the officers time to eat.

In Cat B prisons, association/exercise is only one hour a day, at least in the one I was in ("local" prisons are Cat B and take prisoners on remand and convicted ones as soon as they arrive from court, and convicted prisoners stay there until the Offender Management Unit categorises them and there's space available in a prison of the right Category).  I got lucky - I was categorised as C and moved to a Cat C prison after only a week.

Probably you've never heard of Categories - in male prisons they go from A to D according to escape risk.  A is for the worst of the worst, terrorists, child killers, that kind of thing, and it ranges down to D, open prisons.  Most convicted prisoners will end up as Cat C - "unlikely to make a determined attempt to escape" - so modern prisons of this kind won't even have a wall around them, just a very high fence.

You will also need to be able to break up fights and restrain prisoners (you get taught restraint techniques during training) and do cell searches.  It might be decided to search a cell where the occupant(s) are suspected of having something they shouldn't, usually drugs, and that'll be hard physical work.  Or even do that to the whole wing.  I never had my cell "spun" (to use prisoners' jargon) but I know it takes a while to do it thoroughly.

And are you prepared to move house to where there's a vacancy?  Mum had a friend years ago whose husband was a prison officer at Wormwood Scrubs, and then he got transferred to Chelmsford.  So the family had to move to Essex.

Then there's the night shift.  All the prisoners will be locked in but you still have to do regular rounds and look in all the cells to make sure nobody's escaped.  And I've known some older prison officers who weren't particularly athletic.

If you get the job, may I make a plea?  I've come across prison officers who seem to just not like prisoners, and that is no good to anyone.  Yes, a lot of prisoners are complete arseholes and deserve all they get.  But don't judge too soon.  I particularly appreciated one officer who said "I can't do that for you but I can do this instead" and took the time to explain, knowing I wouldn't throw a paddy.  This was also the same one who said "sorry I have to put this guy in with you but I have to just for tonight".  I was in my mid-40s at the time and I wasn't going to get on with this early 20s guy just off the drug rehabilitation wing.  But next day, he did a swap with another prisoner who had the same problem and now all 4 of us were happy with who we had to share with.  Which surely made life better for the officers too.  It's an odd thing - prisoners in their 20s mostly act like spoiled toddlers but suddenly "grow up" about the age of 30.  My new cellmate and I agreed on this - we didn't want to share with anyone under 30 again!

Honestly - cell-sharing is worse than being married because you can't storm out and spend the night on the sofa if you have a row.

A lot of prisoners didn't like him because he wouldn't stand for any nonsense.  But play ball with him and he'll play ball with you.

they might let you sit if you have a doctors note