I do not want to argue with you if the opinion of the leading UN expert in this matter is based on enough hard evidence. The important thing is:
The United Nations Special Rapporteur on Torture (and other cruel, inhuman or degrading treatment or punishment) visited Mr. Assange with a medical team and then wrote an appeal to the UK Government. The UK government should react with a prompt and impartial investigation. They did not do that. The did not even answer the question of the United Nations Special Rapporteur.
That by itself, no matter if Mr. Assange was subjected to torture, is non compliance with international rules.
Given Brexit this level of "we do not care about international rules and we do not accept the UN as an authority" is unsurprising. IMHO a western nation like the UK ignoring a member of the OHCHR is very worrisome. It shows that the UK government has little interest to participate in the trend towards "good governance".