Never been to Pembrey so can't give you any circuit tips. One thing I would say on your first trackday is to mind your speed. Sounds obvious at first but as the circuit is wider and more open, your sense of speed tends get thrown. You'd be surprsed how much slower it feels when you don't have hedges and trees flying past only feet away from you. This means that on track you may end up going in to corners faster than you think.
Apart from that, enjoy yourself