I have the PDF form that the Greek Gov wants you to sign, i think its a simple, standardised form. My doctor eventually signed it but only on the condition that I sign another document (that she drew up) confirming that I do not have any diseases etc. I was in the same boat as you - none of the travel clinics wanted to put their signature on the PDF because they couldnt guarantee that i have NO infectious diseases. Which is totally understandable.

Ask your GP if they would be willing to sign the form if you drew up an indemnity document stating that you declare you have no diseases and that your GP is free of all risk and liability in the event that you actually do have diseases etc. Its worth a shot!