I live in between all 4 airports (XRY, AGP, SVQ and GIB), I would love to see a connection to LHR from somewhere apart from GIB. Timings with BA from Gib are not brilliant for connections and I head north/south to LHR at least 4-6 times a month, and many a time have to suffer going through Barajas which is not really a pleasant experience. The new flights offered by BA from GIB are welcome, but we need a second daily service instead of the additional 3 days - I don't know whether demand warrents this.
FYI, I read a post recently that Thomson are supposed to start a new service ex LGW to XRY to "compliment" Ryanair's existing STN service.
As C3B has already stated the Costa del Luz is frequented mainly by Germans and Scandinavian's rather than Brits. This is why there is not that many offerings at this time, although SVQ used to have good connections to the UK (might still be the case).