you might have to do it in your own time, as a private candidate of course. eg take annual leave for exam days
but dont expect special treatment or extra time to study. you need to serve just like everyone else.
but if you are in a stay-in vocation, then you might have to leave it til ORD. another way is to take a diploma either part time, or after ORD, full time, then upgrade to a degree in a private school. you have better chance at that rather than trying to catch up with the A level and local uni people. that train has passed because you were too lazy.