I'm not sure that Plymouth is that fog-prone, and the ASW flights to/from Gatwick do seem go in and out of there with almost clockwork regularity on the vast majority of days.
Like any low-cost airline, you'll pay more if you book it at the last minute, but if you book early their 29 quid fare (including all the taxes etc) seems to be quite easy to get. It sure beats a four hour plus trek by road or rail!