I have the feeling that Sky has slightly better peering (more stable speed to US & Asia during peak hours), but the higher speed on VM is more important here, and ping times are generally very low.
What do you mean with IPv6-free zone? Have ipv6 disabled on my PC (for different reasons) but haven't experienced any connectivity problems on either Computers or other devices (which should be able to use IPv6). IF you mean missing availability of ipv6, I don't think that there are any pages you can't see on ipv4?
And there won't be while ISPs are lagging in their adoption, meaning nobody can set up a IPv6-only site if they expect to be accessible by everyone.
Also, there's more than sites: an IPv4-only client can't connecting directly (P2P) to other clients behind carrier-grade IPv4 NAT, which leads to more centralized systems (and which give an advantage to large companies over more independent developers and open source groups).
These ISPs are holding everyone back, hence the site submitted in this thread.
It's unfortunate for people with more technical knowledge, but most people don't have that, and there is point protecting them from attacks (even if it's their fault that they didn't update).