Although anything over 14 days detention is served in Colchester by some units, particularly units with no guardroom, using civvy security etc. I would expect full blooded regiments/battalions who run a full guardroom with Regimental Police etc. to take care of their own.
The Guards particularly do things their own way! They, for instance, wear shoulder flashes on their No.2 dress, have their own unique way of doing drill and many other idiosyncrasies that I can't bring to mind at the moment, so I would expect them to run their own detention up to 28 days still and it wouldn't be pleasant! I stand to be corrected, any Guardsmen out there?
This link;
Military Corrective Training Centre (MCTC) - British Army Website makes interesting reading, if you want to know what goes on at Colchester and definitely says it is used for 14 days up to 2 years detention.
Hardly seems worth the trouble for taking someone there for just two weeks!