Sky charges more for 084/087 calls (what used to be BT's un-discounted rate) so it's best to prefix any 084/087 calls with the CPS override code 1280 to send them via BT.
BTW, there have been no such things as 'local' and 'national' calls since 1/7/2004 - when BT abolished the differentiation and started charging the same for all calls to UK 01/02 numbers regardless of the point of origin.