I've done a total of three track days so far, all via PCGB. Two at Castle Combe, one at Donington.
I can highly recommend the PCGB days for a beginner. I genuinely felt that there was no competition, that people did not mind me going at the speed I felt comfortable with. Part of that is the instruction given in the briefing as to how to be overtaken, as well as how (and where) to overtake - that kind of gave me permission to drive at "slower" (cough) speeds even though there was a GT3/RS behind. The confidence that they are not going to suddenly appear on the inside while you are negotiating the corner is crucial to getting the hang of it.
My first day was at CC and I would recommend the track for a beginner. Not too much to learn by way of geography, but very engaging nonetheless. Ask for an instructor early on in the day - they were very understanding when I did, and it helped a lot.
When are you thinking of going? I'm just trying to make my own mind up...