To add to the thread, most of the PP lounges in the UK are third party owned (with the odd exception) and are accessible by paying a fee. The major operator of third party lounges is Servisair/Globeground and you can buy access online at their website, before your visit. Typically it costs around £17 for three hours but you can stay longer if it's busy i.e. they don't spot you!

I'm fairly well covered with Amex Plat. and its complimentary PP membership and bmi silver card for access to BD's lounges. The only snag I ever have is on the odd BA flight - no lounge priviledge in T3 at MAN for example.