I think only the big 6 block much of the high court ordered piracy/copyright stuff.
At 1 point those 6 ISP's were BT, Virgin Media, Sky Broadband, Plusnet, TalkTalk and EE).
Zen don't block sites on the high court blocking list.
The courts deemed it too costly for smaller ISP's to have to block.
Originally only ISP's with over 400,000 subscribers (the big 5 at the time?) had to apply the high court block lists but that's likely changed a bit over the years.
I helped the neighbour switch to Zen to bypass the iptv blocking that was going on a couple seasons ago during premier League matches.