It depends on the prison but in some your allowed a couple of sets like two pairs of pants,two jumpers (no hoodies or football tops though).But some prisons don't even allow that and it will all be prison issue.
I've just had a friend get out of Aultcourse.His advice is to just keep your head down,he's a quite person and he never had any bother.He says it all depends,but In some you can get single cells.It might be worth speaking to the nurse and explaining about your anxiety,and this might get you a single cell.