They're talking rubbish, Home Office Approval is for people who are here on a different kind of visa, like student etc. You have already been given permission to marry in the UK because you were issued with a fiancee visa, the purpose of which is to allow you to come here and get married. If I were you I would find out where your Local Area Health Authority office is and pay them a visit. Ask them if they can give you something in writing to show that you are legally entitled to register with a doctor here in the UK.
Iain.