It could be that O2 run mobile browsing via a proxy to try and improve the user experince.
Vodafone do this, it screws up some sites, on Vodafone there is often (usually?) an extra link at the bottom of served pages letting you seee the original page. might be worth seeing if O2 offer the same.
Have you tried using the latest opera browser?
FWIW PPrune works fine on my Nokia e71 with the standard browser, I tend to turn off images, the width of the forum posts then just fits the screen.
Memetic