I'm currently moving from sky to BT.
In your case I'd go to sky. The service on ADSL or fibre is excellent. I'm only moving as my discount on fibre (1/2 price for 12 months) ended and moving to BT gets me a better router for a similar price (when sainsbury's vouchers are considered too)
When your discount is due to end, just call up and see what they can do for you. I've had discounts for the last few years.
Sky Fibre Unlimited
Ashington (Northumberland) Exchange