Hi Ricky
In answer to your question, you can switch anytime you want, but because you're thinking about switching from cable back to the BT phone network it's going to be more hassle and it may take longer than usual.
For a start you're on a package with a total bill for all three services for £55, as you know NTL are offering the same service now for £30 a month for new customers, but that doesn't mean you can't get it....
i) Check to make sure you're no longer inside your contract period, if you are it will be more costly and more hassle to switch, if not then you're in a better position.
ii) Speak to NTL and say you're planning on switching to Sky, say they're offering you:
Sky Digital - 2 Variety packs - All the Sports channels free for 6 mths - £15 per mth - free box and installation (Sky + for extra £55)
Sky Talk - free calls UK landlines - £5 per month - free installation and equipment
Sky Broadband - up to 8 meg speed, 40Gb download cap - £5 per mth - £20 installation and free modem
TOTAL: £25 per month (don't forget BT line rental £11) - £36 per month
So basically you'll have all the Sky Sports Channels, a faster product with more download capacity more channels and free UK calls anytime for £19 less a month than what you pay right now.
Threaten them and say if they don't offer you the NTL 3 for £30 package you're off, (maybe ask if they can chuck in the Sports Channels on the cheap). After they make you an offer consider your options and choose between the two.
Good luck.
Jason