Is your favourite site blocked by your UK ISP?
https://www.blocked.org.uk/
There's at least two things going on here.
1) Sites blocked by "Adult Filters" that are defaulted on by the major ISPs but can be turned off. This might be an obvious porn site but could also be collateral damage. For instance These have included a website selling Porsches, a Jezebel article on post pregnancy care and the women's rights website SheRights.com
2) Sites blocked because somebody, somewhere disagrees with them. This is often because the site has something to do with sharing paid entertainment such as the Pirate Bay or New Album Releases. These are blocked regardless of whether you've disabled your "Adult Filter".
You can often get round these blocks via automated proxies like the one at https://immunicity.org/
Won't anyone think of the kittens? Thank goodness that Claire Perry and David Cameron are thinking of them for us. /s