[petercooper]

Others have already posted, but the biggest domestic British ISPs block a variety of things, like SciHub, Libgen, Pirate Bay, or Anna's Archive. Coverage varies a lot though, so I assume ISPs have some discretion and enforcement is patchy.

[squidbeak]

This isn't the case for me with Anna's Archive or Sci-Hub. I use the biggest ISP, and both are fully accessible.

[petercooper]

Implementation of this stuff must be very patchy then as both are off on my top 5 provider until I use a VPN. Which makes me wonder why any of the ISPs bother blocking at all, if they can just pick and choose?

[sceptic123]

I'm assuming BT? If so then their blocking is DNS based and if you are not using their DNS then they will block these sites