Two years or less - Motor Cycle Training Centre.
Anything more - civvy clink.
That is, unless the MOD can't countenance having prisoners guilty of the particular offence(s). Usually the sentence itself would be longer than two years so not an issue, but there can be mitigating circumstances.
I knew a chap a long time ago - later in his career, after I had left, he got done for kiddy porn. He only got 6 months but didn't go to Colly. I think the MOD & civvy police figure out where's best to try the nonces and such.