From
http://www.oxfordgroundschool.com/oxfordgs.htm
"There are three appropriately structured distance learning courses, dependant on which licence you currently hold. The courses are split into two phases - each one culminating in a two-week "brush-up" at Oxford Aviation followed by the JAA exams."
Don't quote me, but I believe if you have a PPL you must take the JAA650 course, if you have a CPL you take the JAA450, and if you have other experiance (i.e. military flying experiance) you take the JAA300 with agreement from the CAA.
The number 650/450/300 relates to the number of hours home study you must do to complete the course. This is measured each time you hand in an assessment paper. The school keep a record of the number hours study you have done to ensure you do the minimum.
N.B. The minimum hours are really that - minimum. Expect to study for considerably more hours
I hope this helps. I suggest you email Oxford directly and they can tell you everything you need to know related to their distance learning courses.