For London you have to book the medical and the interview yourself. The medical i believe you can book by ringing up knightsbridge. Once you have a medical appointment date and since you completed the DS160 online you would be good to go to book the visa appointment at the embassy as well. The booking for this is online and if i remember correctly they mailed my husband instructions on how to sign up etc. Just make sure you book your embassy appointment at least 7-8 business days after the medical. They recommend 5 business days (which we did) and my husband's medical hadn't arrived yet. You can still do the interview without it but it's better for them to have it.
