Can I cancel sky and rejoin?

Bungaroosh

Do a bit of research and look up some packages with competitors that you would be willing to switch to (and look up what is offered for this on the cashback sites).
Phone Sky and say "(e.g.) BT is offering X, Y and Z for £xx, so it looks like I don't be much choice but to switch".
Try phoning a few different times be speaking to different people (over the course of a week or however often you can face having one of these chats).

If they are still not offering anything sensible, then start a transfer (e.g.) to BT. After a couple of working days, phone Sky to "check that the transfer is going through". At that point they will throw a kitchen-sinkful of offers at you to get you to stay. Then you can easily just phone BT and say you're staying with Sky after all and that you'd like to cancel within the cooling-off period.