I never seem to get that far. Don't you actually have to threaten to leave and effectively call customer services' bluff to get put through to retentions? Is there a direct number for them?
You just go through the phone system and choose the "thinking of leaving" option. Then you get through to the UK customer service team. Basically tell them that you're thinking of switching to Sky/BT because of all their new customer deals means you'll get a much cheaper price, and what deals can they offer you to stay.