The following is a non-exhaustive list of websites and domains blocked in the United Kingdom — either by order of the High Court (for example the Pirate Bay, Kickass Torrents, etc.) or of ISPs' own volition.
As of 2019, it appears to still be the most comprehensive list available. Most domains featured on this list are likely blocked by all of the 'Big Five' major UK ISPs.
Updated semi-frequently — most recent update: April 2019.
Please note, these lists do not include domains blocked by the IWF, as they contain abusive imagery.