I think not, as you say yourself, mental derangement can make a defendent incapable of possessing mens re(a) - which is exactly my point. If a disease can be shown to have removed a person's ability to act in a moral way, he can not be culpable in law.
Not quite correct.
Some mental derangements can affect mens re. But in particular, a lack of a moral compass is does not have that effect. If your reasoning does not allow for this, then it is wrong. Further, I'm absolutely sure that US and UK law are identical on this point as we inherited this bit of common law from you guys.
If a disease can be shown to have removed a person's ability to act in a moral way, he can not be culpable in law.
In some jurisdictions, yes. But not because of a lack of mens re.
For example, a person who knows that a gun can make a hole in a man's head resulting in death, but does not appreciate that doing so is wrong can possess the mens re to commit murder.
On the other hand if a persons derangement makes him think that Bob is a reptile, then the person cannot have the requisite mens re to murder Bob.
In some jurisdictions, the lack of moral compass may allow the person to escape punishment
despite having met the elements of a crime (including mens re).
In other jurisdictions lacking a moral compass would not be sufficient to escape punishment. In the US, every state picks its own standard, and the modern trend is to make if increasingly difficult to use insanity as a defense.
Which brings me to some reasons why the distinction between mens re and the insanity defense is significant. In the US and in the UK, mens re is an element of the crime that the state must prove beyond a reasonable doubt. On the other hand an insanity defense must be proven by the defendant. The defense has to warn the state sufficiently far in advance of trial that it intends to prove insanity. On the other hand, it is always in season to dispute the state's evidence regarding mens re.
In the UK at least, Fred would have been fairly quickly judged incapable of even making a plea
That doesn't address the issue at all. There are many possibilities for avoiding punishment for a crime and only a few of them deal with mens re.