Foreign national Prisoner question...?

British prison Categories are all about escape risk. An attempt to escape from a Category C prison would certainly result in recategorisation to Cat B. Whether the prisoner is a foreign national or not, it's exactly the same. The definition of Cat C is "prisoners who cannot be trusted in open conditions but do not have the resources and will to make a determined escape attempt".

And that is why a recently built Cat C prison won't have walls around it, just a very high fence. That ought to be enough to keep the prisoners inside, and it will be according to that definition. There are some Cat Cs with walls but that's only because they are older and got built with them, or they used to be B. For example Brixton was always a B but recently it's been "re-roled" as C.

Obviously if a Cat C prisoner actually does make a determined escape attempt, they're in the wrong Category and will be moved to B, "prisoners for whom the highest security conditions are not necessary but for whom escape should be made very difficult".

Extreme misbehaviour might do it as well. Certainly any serious actual crime committed in prison would get the prison's Offender Management Unit looking again at where they should be. Murder another prisoner and you bet your boots we're looking at Cat A "for whom escape should be made impossible".

Just for completeness, the remaining Category is D, open prisons. You could abscond from one of those by just walking out of the door (it's not called escaping because you don't actually have to break out), but the prison will soon notice when you don't sign in the required five times a day, the call will go out to all police forces, ports and airports, and probably they'll catch up with you inside of a week. When they do, that's a return to Cat B, no question. Apparently open prisons experience one abscond a week on average, and that's what always happens on recapture. The real utter berks who try this go home - of course the local constabulary will be waiting on the doorstep!

in what country

They are moved if they are at risk of escaping or they have misbehaved in some way. They are treated the same as British prisoners.

I worked in a Cat C.

UK

EXACTLY the same as if they are not a foreign national, whatever those are.