Hi Siggsy,
I currently use Sky Broadband and it costs me £7.50 each month for their "up to 20mb" package. As you're already a subscriber to their TV package (and depending on what kind of service can be provided in your area) they should offer you this price too, or at least something similar.
Sky should provide you with a wireless router anyway, normally a "Netgear" piece of kit. Now, I don't own a PS3 (so I may be wrong), but you can connect wirelessly to your router (as wilkesneil suggests) or you can connect an ethernet cable directly from the router into the back of the PS3. I have heard that some people struggle to connect their Netgear router wirelessly to the PS3, so make sure you have a lengthy ethernet cable handy just in case the wireless connection is dodgy.
If you have any other questions then let us know, but I suggest you get on the phone to Sky asap to ensure you get your package delivered before Christmas, and that they turn your line on in time. As wilkesneil said, don't let them flog you the ultra-broadband packages. Up to 20mb at £7.50 should suffice.
Let us know how everything goes!
Ed.