All prison work on rigid timetable. A typical Cat B prison will start the day at 8am when prisoners are taken to work or education, with those not involved in these activities locked up by 8:30am. Lunch is at noon and afternoon activities start at 1:15pm . Evening meal is at around 5pm after which these is a period of association between 6pm and 7:15pm. The regime at weekends and bank holidays start later with unlock at 8:45am, and final evening lock up at 5:15pm. Look at the details for the prison you are likely to be sent to check what their core day is.
In a Cat D prison you may well be unlocked all day, and are only required to attend roll calls 3 or 4 time a day, again look at the specific pages on your prison for details