In this case, every site they can identify. Broadly, UK law is restricted to UK companies for material which is illegal to post. So libelous sites hosted outs

In this case, every site they can identify. Broadly, UK law is restricted to UK companies for material which is illegal to post. So libelous sites hosted outside the country aren't censored, because the libel isn't 'located' in the UK (yes, this is a tortured analysis). But the UK has more recently established a China-style firewall that restricts access to sites hosted anywhere, based on where you view from. Originally, it was implemented to block access to torrent sites. And of course, at the time everyone swore "it's just the torrent sites, there will never be a slippery slope!" But now it would age-gate pornography viewed from any UK IP address. Well, the content they can find anyway - they'll have to either run a blacklist (catching only large, paid sites) or a blocking filter with a whitelist (blocking all kinds of legitimate content - usually this ends up blocking sexual health sites). And yes, Tumblr is hopeless, there's really no way to know what a given page will have on it. So it's the authoritarian, cut-off-access bit, but it still won't work.