It actually does make sense to blame the UK for this. UK law (Digital Economy Act 2017) says that ISPs are legally required to offer content filtering to their customers, turned on by default, although customers are allowed to request to opt out of it. So, this ISP is legally required to impose content filters unless a customer specifically asks them not to do so.

Now, you can say the real issue here is not the filtering itself, but the decision to include this particular website in the list of sites to be blocked. But the reality is, filtering decisions are inherently subjective, mistakes are inevitable, so I really think if someone ought to be blamed, it is not the company legally forced to do the filtering, but rather the law which forces them to do it. Also, the company wants to spend as little money on the filtering as possible, since it is a pure cost centre – and if customers don't like it, they can opt out, and then either have unfiltered Internet, or filter it themselves – meaning there is no incentive for the company to do a good job of choosing which sites to include

I'm a parent (of a 9 year old and a 4 year old), and I think it is totally good parenting to seek to limit your children's access to online material they are too young to process. But I think the right answer is a free market of parent control solutions (both commercial and open source), with parents being able to choose the solution which works best for their family – not government-imposed network-level filtering, even if it is an opt-out. Parental control is best done at the endpoints, not in the network.