The London consulate says medical should be 5 business days before the interview. It also says to reschedule the interview if you cannot complete the medical accordingly. Having said that, if you choose to attend the interview before the medical, the consulate officer can just conduct the interview, then place your case in administrative processing status until the medical exam has been reviewed later. Good luck.
About the Medical Examination for an fiancee immigrant visa. (usembassy.gov)
"All applicants, regardless of age, are required to complete a medical examination performed by an Embassy approved physician prior to their visa interview. We cannot accept medicals performed by any other physician.
When scheduling the appointment, please note that it takes a minimum of 5 work days for the results of the medical examination to be couriered to the Embassy.
If you find that you are unable to attend a medical examination prior to your visa interview, you should reschedule your visa interview for at least 5 working days after your medical appointment. Please schedule the earliest available medical examination."