Virgin media are the top on the internet list in the UK from my personal view and many others.
You mentioned them installing the new fibre in the green box at the end of your street, sounds to good to miss, if BT offer you a better deal I can't see it being as fast as virgin but I think it's not far behind, sky on the other hand are mostly aiming for providing the best tv service where as virgin lacks abit behind, we have virgin tv aswell and it's not as good as sky, so theirs two ways, stick with sky and get the BT internet or skip sky and go for virgin package with tv+phone+internet...
this is just from my personal experience, my answer is go for BT because of the sky tv which I'm assuming you have?