@timber321 my experience with the US Embassy was super easy. London is one of the best and most coveted international assignments, so you tend to get the best government workers. I’ve had to deal with the Embassy a few times and always felt like I was in a weird governmental utopia, where things work and workers were friendly. The new building is modern and gorgeous, and there’s even a coffee bar in the waiting area. I’ve actually never seen a nicer government building. It’s a nice change from the other US embassies, which tend to look and feel like maximum security prisons…
Like you, I booked an appointment at the Embassy a couple of weeks in advance “just in case.” You will need to fill out DS-11, AND also fill out DS-64 (lost/stolen passport). I wasn’t sure about DS-64, but the embassy staff told me I needed to fill it out. I explained the situation that it was missing v. definitively lost, and they said it was correct to report it lost at HMPO Peterborough. Your appointment will go faster if you fill out this form in advance. It’s also better to get a US-sized passport photo in advance, though there is a photo booth there. I brought a whole host of documents, but they just asked to see another US government-issued photo ID. I showed my US driver’s license, which I keep renewing (using a US address). They didn’t ask to see proof of flight or anything that backed up my story of submitting the passport to HMPO.
By reporting the passport lost/stolen, your existing passport will be cancelled/invalidated. There’s no way around this (I asked). The embassy officers were extremely understanding, far more than I thought they’d be. The two officers I dealt with both said “the British lose passports all the time.” One was familiar enough with the process to know that they mail the documents back separately (and he offered his opinion that it was silly). I was apparently the second person that day in this situation, at 7:45 a.m…. I got a temporary passport valid for one year on the same day, in less than an hour actually (and less than 15 minutes after I filled out the DS-64 form).
After hounding the passport office almost daily and getting my MP involved, I got my US (and Italian) passport back 9 weeks after I received my British passport and 13 weeks after submitting it to HMPO. I now need to return this passport to the embassy for cancellation, along with the temporary passport, which the US Embassy will replace with a full validity 10-year passport (for free as long as I do within a year). You can use the same photo if you replace it within six months. The process of exchanging temporary for permanent allegedly takes three weeks. I need to go to the US again this month, and I decided to wait to be safe even though I had six weeks between trips. I’ve had enough passport drama this year!
The temporary passport has no chip, but I was (surprisingly) able to use it at a Global Entry kiosk. I had updated my details online in advance.
Hope this helps!