[jimnotgym]

They may be voting no to the Brexit deal purely because they don't agree with it.

But you are painting this as a binary situation of 'The Deal' vs No deal. There is a third option, withdrawing notice to leave.

Parliament can choose to remain without any kind of referendum. In fact there was a case heard by the ECJ today to clarify this very fact. The UK's option to remain was undisputed, and the remaining legal arguments are simply over whether the remaining 27 states have to agree or not. The ruling is due at a later date.

[Findeton]

That's a technical possibility. But in reality, MPs just don't have the incentives to do that. They just wouldn't get elected next time. It's unthinkable that the UK would choose to remain without a referendum. And the thing is that as of now, a referendum (and I'm not even saying what should be asked there) is also not a politically viable option.