Season 1 is dull as hell! After the first few episodes, I was so bored that I fast-forwarded through any scene that didn't have Thelma in it. The main villain doesn't seem to ever do anything except stand around in the background and ominously stare at Cassie.
I love Thelma. She's such a fun sidekick, although the rules regarding her ghost status are a bit hazy. It's like she's depressed because, now that she's dead, there's no possibility of a sexual relationship with Cassie because she can't touch her. However, she can touch objects, mostly when no one else is watching. However, there are often scenes where she'll be eating a bag of pretzels or something in plain view of other people at the school, and yet no one seems to notice a bag of pretzels being suspended in thin air. So I'm really not sure how this is supposed to work.
Still, I love how well ghost Thelma has integrated herself into Cassie's life. It's easy to forget she's dead. There's a scene in early Season 2 where someone notices that Cassie seems a bit troubled.
"Is it Thelma?"
"No, Thelma's fine. She's actually been very good late...ly..."
Season 2 makes some jarring cast changes but generally seems to pick up the pace and make the episodes much more engaging. Unfortunately, Season 2 isn't available on DVD in the U.S. (except for the 1st 4 episodes of Season 2 that were tacked onto the end of the Season 1 DVD set).
IIRC, Hex wasn't a BBC series. It's clearly British but I thought it was on ITV with A&E distributing the American DVDs.