BT broadband (as opposed to fibre) is reliable but slow. Speed can fluctuate wildly. That said, Sky would be the same as they use BT's infrastructure. If there's a cost saving then I'd switch. I'm actually looking at swapping the other way (BT to Sky) on the basis of cost, but that's because I use the phone a fair bit and I'm finding I'm spending almost £50 a month for everything with BT.