I definitely agree with the judgement here.
Firstly, I don't think the size and space he lives in is relevant to the points in the case. Denial of interaction with other humans (which is known to have a severe impact on mental health over long periods), and sleep deprivation. Sleep deprivation being specifically recognised as a form of torture. So definitely this meets the test of a prohibition on torture and cruel treatment.
Now, the European Convention on Human Rights prohibits such activity under Article 3 of the convention. There is no opt out. And just in case anyone thinks this is just those silly liberal Europeans, this is also the position of the Universal Declaration on Human Rights, the Convention against torture, and in some cases, the Refugee Convention. All of these have global applicability.
When it comes to rights like these I really dislike the idea of an opt out just because someone is a criminal, or because of XYZ. These are all lines drawn by the state, and primarily the state is who these rights are designed to protect you against. I also especially feel that it is important for us as a society to not stoop to the level of violating peoples rights because we think they are somehow undeserving of them. "All Humans have rights apart from those that don't" is an argument that can be applied to serious criminals, or not so serious criminals, or those that are committing crimes that society later realises shouldn't be crimes (like the treatment of homosexuals in the past), or because they are black, or Jewish, or whatever reason is convenient to separate into the "deserving of rights" and the "not deserving of rights."
The final issue here is the one of who imposed this penalty. Judges imposed a prison sentence. Public officials imposed the isolation and sleep deprivation. Judges are subject to scrutiny and appeals procedure in a way that the prison officials are not. I am not comfortable with the idea that a faceless bureaucrat gets given the power to determine that an individual is not deserving of these rights, and can be given treatment that degrades their mental health.