Thickest wooden floor underlay?

[SilverPenguin7]

I'm going to be buying my wooden flooring this wednesday for laying down next sunday. However as I am going to be running cables under it I'm not sure how thick the underlay needs to be. All the stuff I have seen in homebase etc has been 6mm thick and whilst thats fine for speaker cable I am hoping to run a couple of power cables under it as well (one of which will be an extension lead which is fairly thick). Would doubling up on the 6mm stuff be ok or is there underlay out there that comes thicker than 6mm as standard? My only worry with doubling up on the 6mm stuff is going to raise the floor slightly too much meaning there will be a lip where it stops and the next ajoining rooms starts

Any advice would be great