<Moved to "Other ISPs">
Sky has an 80/20 product (Sky Fibre Pro), just as BT has...
But it's not advertised on the Sky website as prominently as the standard 40/10 Sky Fibre Unlimited... You have to dig for it, and I believe you can also only order it by phone.
http://www.sky.com/shop/broadband-talk/fibre-optic/
If Sky Fibre Unlimited is available in your area, and you're near enough to the cabinet for BT Infinity 2 to be available, I see no reason why Sky Fibre Pro would not be. They all use the same Openreach fibre from the exchange to the cabinet, and the same Openreach copper from the cabinet to your home.
However...
Although Sky's 40/10 service is cheaper than Unlimited BT Infinity 1 (£20 for Sky Fibre Unlimited versus £23 for Unlimited BT Infinity 1), their 80/20 service (Sky Fibre Pro) costs £30 compared to £26 for Unlimited BT Infinity 2. Plus of course with BT you also get BT Sport.