Well the Q400 will cruise at about 25,000' on the way to Perpignan. If I remember correctly (I haven't flown it for a few years) the cabin altitude at that height will somewhere between 4-6000'. Probably a bit higher than on your trip to Jersey but not as high as your trip at 38,000', which would have been up at 8,000' cabin altitude.
Not being medical at all I can only surmise that your O2 levels will be somewhere inbetween the two extremes that you mention. However, the flight to Perpignan is about 1 hour 40 minutes so depending on how your problem manifests itself that may have some bearing on it.
Hope that helps.