First of all, I've never heard someone say "not half good" unless it is used in the context Trisia used. In America, "not half" is used with the negetive, and almost always with the phrase "not half bad". I'm not sure about BE, but you seem to use it completely differently. My sister spent some time in Wales for a year and picked up many BE manners of speaking, and I've noticed that she uses "not half" just like you, and everyone mistakes her meaning. It depends on the context of the writing, but I'm almost certain that this means that it's good. Not amazing, but certainly better than expected. This is commonly used with food.
The dumb guys look at a luxury mansion and think it needs improving (I think the first speaker is sarcastic though):
-- Boy, this is quite a shack, huh, Har?
-- Yeah, but I bet if you pulled the weeds off the walls and slapped up some aluminum siding, it wouldn't be half bad.
Dumb and Dumber To, movie