You will need a copper phone line into your premises. At present this is normally provided by BT, but Sky propose to site their own equipment in BT exchanges to connect to this line. This is known as Local Loop Unbundling.
Although you would no longer be paying BT to rent the line, who would maintain it and repair faults is not clear. If it was subcontracted to BT engineers, it is tempting to suggest their hearts would not be in it!
Be aware that Ofcom has announced the lifting of restraints on BT's charges from August 1st
click here, allowing them to compete with offers such as Sky.
For my money, I would stick to BT until the picture becomes clearer. 'It's your call'.