socket box in skirting board?

[thegreenkiwi]

The socket boxes in my house have been mounted on the wooden skirting boards which are about 20cm high. I want the boxes to be flush, but the mains cables don't have enough slack to install the box in the wall above the skirting board.
Are there any regulations that say i can or can't install the box flush into the skirting board? and if i can does it matter if i use a metal or dry-lining back box?
Any replies would be much appreciated.