There doesn't appear to be any reliable evidence that Assange is being held in solitary confinement. The original source for this seems to be his father, who claimed several months ago that he was being held in solitary for "up to" 23 hours a day.
There have been photos leaked by another person showing Assange looking relaxed interacting with other inmates. The leaker claimed that Assange was popular among the other prisoners.
In fact, Nils Melzer himself has confirmed Assange is not in solitary, but is apparently being "tortured" for other reasons. It is not clear why these reasons don't apply to every other prisoner on remand in Belmarsh.
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".
There have been photos leaked by another person showing Assange looking relaxed interacting with other inmates. The leaker claimed that Assange was popular among the other prisoners.
In fact, Nils Melzer himself has confirmed Assange is not in solitary, but is apparently being "tortured" for other reasons. It is not clear why these reasons don't apply to every other prisoner on remand in Belmarsh.