That is the million pound question!
When BA tell you it is by invatation only they are correct. I dont think that any they have a threshold to join it so you can't earn enough points. Merry as we know is one as he does a shed load more flying than most of us. My 3000+ points this year will be nowhere near enough to gain the "P" card. I know of two other Prems, they both do some Premium travel but not what I would call a hell of a lot, they both do however have very high positions in a company that spends one hell of a lot of money with BA.
What do Prems get over Gold? Too be honest i don't know. I am sure they will get a seat on a flight in their choice of class. Award availabiliy is probably better?
I am happy at Gold as I have been for a few years, I dont think Prem is something I need to worry about as I get looked after most of the time anyway.

Any Prems want to add some facts to this? I am only guessing at the criteria and benefits. Any way I like the secrecy behind these levels...