You don't need an S-Plan PIN to get S-Plan pricing, but it does serve as a good conversation starter. I got a pretty good deal on a Sport Auto with Popular last weekend - under $18k+doc, reg and sales tax. Well under Edmund's TMV pricing.
I started by emailing many of the dealers within 150 miles told them I have an S-Plan PIN, and what I wanted - Sport, with Auto, no Popular / CD changer, no color preference. What have you got? Some didn't want to play, and didn't reply, some quoted prices that were clearly not S-Plan pricing and were told so, some came back with reasonable quotes. Some will need a nudge to include a a break down, don't assume all are including destination, ask them what they charge for doc fee, etc.
All in all this took me a week of emailing - I never visited one dealer in this time. When I had a price I liked, I walked in with a check, they countered with a slightly better loan, and an hour later all the paperwork was done.
In hind sight I think I could have saved a few hundred more, waiting till closer to the end of the month, beating them a little more, but I was pretty much down to one dealer and I didn't want to risk losing the deal - I felt I'd done pretty well getting it to under $18k. And this was my first time buying a new car from a dealership.
To get S-Plan you need a connection mine is through my employer, but it doesn't mean the dealer won't try taking you for a ride.