Following on from my reply of last night, I have just been looking at the CAA statistics for Luton.
As you haven't said when in the year you travelled I have taken February, June and November - thus covering the 3 seasonal operating periods. In all of these months no passengers are recorded as flying to Prestwick, only to Abbotsinch (Glasgow International). I did, out of interest also look at the Stansted routes and note that Ryanair pulled off the Stansted/Prestwick route from the start of the Winter 2011/2012 season.
This therefore leave you with the choice of either easyJet or Ryanair - I excluded Wizzair in my reply last night. In February the passengers from Luton were 21,106, in June 24,587 and finally in November 24,008. The fact that there is minimal difference over these months I suspect that the Luton route is/was only operated by easyJet.