Signage is Arriva, so let's assume it's recent and therefore Welsh. Other possible operators are:
Virgin (West Coast) which would place it on the North Wales Coast. The main hourly service clue I think would place it west of Llandudno Jn, the station looks to be unstafffed but with substantial buildings and doesn't look like Llanfair PG, so we'll discount that.
Cross Country, which would place it between STJ and Gloucester. There are plausible answers there.
FGW, which I think would place it on West Wales branches.
Let's go for Lydney. Open floor if I'm right.