The 5 did not consent because:
the only interesting reflection I think, based on clarification exchanges I've had prior to some people consenting, came from the too vague formulation:
"I would engage in a conversation about my experience with NHS or equivalent"
For non native English speakers this "would" is not so conditional as intended – so they may think they are asked to, or need to, and if they consent they commit to something.
Upon re-examining the list of people who did not respond, we found several that we might have a direct channel to. So we are doing one last attempt to transform them into "consent given". Stand by....