I should clarify that my previous post - written in haste - was slightly misleading.
The test would not actually be whether the reasonable person would consent to sex with someone with a negligible or zero risk of HIV transmission. The reasonable person test actually applies the other way around.
So the question is whether the reasonable person, being HIV+, would know that the person they are having sex with would refuse consent if they knew about their HIV status.
In asking for evidence that they would not, in most circumstances, I think you misunderstand the concept of a reasonable person. It is not based on some quantifiable evidence of what people think. It is a legal concept built on logic and reasoning, and assumes the reasonable person is in possession of the appropriate evidence and facts.
If Jack has sex with James, knows that he is HIV+, but also knows that his viral load is undetectable (because Jack knows that at his last check up it was undetectable, and he knows he has taken his medication as prescribed since then) then there is no basis at all on which to think a 'reasonable person' would conclude that James would not consent to sex if he knew Jack's HIV status (assuming consent is present for all other purposes).
But of course don't take my word for it - I am describing the law as it is now. If the contrary were true, and a reasonable person would be expected to know their sexual partner would not consent to sex if they knew their HIV status, then that would constitute rape under England and Wales law. But the CPS has not prosecuted a single case that I am aware of based on this principle.
So the CPS certainly doesn't seem to consider that the HIV+ person knows, or reasonably ought to know, that consent would not be given if the sexual partner knew about HIV status.