Hi Jon,
Yes, it has been flippin hot, even for those who live here full time.  Anyway, here's my two pennuth.
I've used both airports quite a few times and on balance, I'd go for Valencia.
Easyjet (as far as I can make out from their timetables) do a Saturday flight during winter (October onwards) to/from Bristol and daily to/from Gatwick.
The new car parks at Valencia are very close to the terminals.  The distances are Barcelona 170km, Valencia 197km BUT there's only one toll on the Valencia route and 6 or 7 on the Barcelona which makes it a lot cheaper and probably no longer timewise.  The route to Valencia is straightforward and on the way back to Tortosa, there's only one point where you need to be vigilant near Castellon where you need to make sure to follow the signs for the AP7 and NOT the A7 else you'll finish up in the middle of Castellon.  It's quite a scenic route despite being motorway most of the way.  I'm not going to get into discussion on the relative merits of Ryanair and Easyjet but if you want anymore info (detailed routes or whatever), just shout.
For what it's worth,
Bigyin