Newquay would make sense, but when did they visit Prestwick? If you look at where they have been all of the airfields would be logical options for them to operate into / out of.
If your talking about the use of both these airfields for training by other airlines, you need to differentiate between training like Flybe are doing (that is not the norm) and Base Training for a none ZFT type rating. Both Newquay and Prestiwck are used for Base Training by a few airlines, no doubt for many reasons.