There's a difference between detention and imprisonment. Not much a difference, mind.
This guy (according to arrse) got 21 days' detention. For any non-UK pers, this means 21 days in jail. As it was over 14 days, he would probably have served this in the Military Corrective Training Centre. Which is not quite a prison. But does lock the miscreants up at night and finds tiring military training for them to do during the day.
The prisoners are not paid, therefore they are effectively fined, although the CO can award a fine as well as detention. Periods in detention have to be "paid back" at the end of one's service too, so when the guy gives notice to leave, he would have to do another 3 weeks (paid) to account for his "non-service" over and above the notice period.