It's a difficult one because dealing with Insurance companies is always a nightmare! I can only assume that the reason they want 18 month medical history is to check whether you had any illness or pre-existing injuries which you did not disclose when you took out the insurance. If you did, then this would invalidate the insurance.
If you don't get any joy getting the form in the first place, write to them Recorded Delivery requesting the Form of Authority within 7 days, and that you find the delays incurred already in this matter to be unreasonable. Whether that will actually get them to move or not is another matter, but at least you'll know they've got your request for the Forms, and can't claim not to have received it.
Just keep onto them, that's the only answer. Phone them, e-mail, write to them. Find the name of the person handling your claim and contact them direct.
Good luck x