Can anyone recommend a traffic exchange?

Hi, Can anyone recommend a traffic exchange program where you exchange with other UK sites and also where a visitor to your site only gets a popunder popped at them once in a session, even if the code's on every page? I joined TrafficZap last night but as it's American, I'm not sure what the percentage of UK sites will be in the program.
I'd be grateful for any feedback on this.
Cheers!