Lankyshakes

Thank you! The interview is by far the easiest part 😊 The embassy staff are all super friendly and it definitely doesn't feel like an interview! 

MRV stands for Machine Readable Visa and it’s the visa fee you pay.
You’re ds-160 has a confirmation page with a barcode that’s read by a machine. 
There are so many terms and websites to understand and use it’s no wonder you’re confused.

And since your petition/application will be process via the Embassy..
 
K Visa Applicants –  pay the MRV application fee via the Consular Section’s Visa Information and Appointment Services contractor prior to the visa interview.  Information regarding payment will be provided to you in your visa interview appointment letter.
 
Have you received the appointment letter?

you fill out ds-160, get the number, go to the https://ais.usvisa-info.com/en-gb/niv register, create new applicant, you will need your ds-160 number, pay the fee, book your appointment
 

Sandly, but the appointment letter doesn't provide you with the link of where to pay the fee. Since K1 is half immigrant/half not it can be very confusing with all the information they give.

You don't need everybody on the Internet. Just read and follow the four steps your embassy gives you. 
https://uk.usembassy.gov/visas/fiancee-2/applying-for-the-visa/
 
The link is provided.

First there is no DS-160 fee. Never was.
 
The K-1 visa interview fee is $265.00. Payment can be made following the UK website here: https://uk.usembassy.gov/visas/immigrant-visa-fees/
 
It's amazing what Google and the embassy website will tell you.

The $160 fee is for tourist visas, they also use the ds160 for that

A k1 Visa fee is $265USD not $160, so ignore any reference to $160

Some embassy expect for you to do everything.  After we received hard copy of our case# and NVC informed us our case was now at the embassy, my fiancé did his DS-160, went to pay the $265.00 fee, in 2 days the system said ready.  This is when we finally got information from the embassy to give us packets 3&4 information.  It was sent by email.  He then scheduled his medical & is waiting for an opening to schedule interview. 

Here is where you need to be when asking questions specific to the UK. There are experts over in the sub-forum for your country located here http://www.visajourney.com/forums/forum/99-united-kingdom/  This is where others from your country who are processing or those who have completed their interview and they can guide you on their experience. A TON of information and help there.
 
A fellow VJ'r who just completed their visa interview
https://www.visajourney.com/forums/topic/690372-my-whole-k1-experience/
 
Medical prices including vaccination pricing
http://www.visamedicals.co.uk/us-price.htm
 
Medical questionnaire
https://uk.usembassy.gov/wp-content/uploads/sites/16/2017/05/USA-MEDICAL-QUESTIONNAIRE_IV.pdf

First advice is ask the questions in the UK forum because each consulate has their own way of doing things. 
 
The letter you are waiting on is not needed. It has is a link to your instructions. Here https://uk.usembassy.gov/visas/fiancee-2/applying-for-the-visa/. Scroll down to this:

 
Start at Step 1 today and continue because your case should be there. Did you check your status here https://ceac.state.gov/IV/Login.aspx it probably says "ready".
 
Then call to book the medical http://www.visamedicals.co.uk/us.htm 
Despite the embassy and medical people saying allow 5 days for medical to get to the embassy, it takes longer. Leave a good 14 full days between.
 
Read Step 4 of the instructions so you understand the interview booking process before you jump in.  
 
For your petition expiration concerns, from the Embassy list of documents to bring to the interview:
Petition Affirmation Letter (if applicable)
If your petition expires before the visa interview takes place, you are required to bring to your interview a signed letter from your petitioner confirming that you both remain legally free to marry and that you will marry within 90 days of your arrival in the United States.  
 
 

And as the others mentioned, don't worry about the expiration of the petition. The consular officers have authority to extend the petition as they see fit. 

They won’t contact you any further really.
 
go ahead and book your medical and interview now that you have your NVC number. I hope you got your police certificate. You can also fill out the DS-160 now. 

Petition are routinely extended by the consulate for up to one year 

Go to the UK forum for all things London 

100%
u can submit it the day u get noa2 but most people wait until their file leaves the nvc.
so now u know
 

P3 instructions for the UK
https://drive.google.com/file/d/1M92AwztXRnkFw6xwUF0ZlntJ6D4-eK2E/view?usp=sharing
 
Embassy Page
https://uk.usembassy.gov/visas/fiancee-2/required-documents/
https://uk.usembassy.gov/visas/fiancee-2/medical-examination/
 
London Portal here on VJ
http://www.visajourney.com/consulates/index.php?ctry=United Kingdom&cty=London
 
London sub-forum on VJ
There are experts over in the sub-forum for your country located here http://www.visajourney.com/forums/forum/99-united-kingdom/  This is where others from your country who are processing or those who have completed their interview and they can guide you on their experience. A TON of information and help there.
 
A fellow VJ'r who just completed their visa interview
https://www.visajourney.com/forums/topic/690372-my-whole-k1-experience/
 
Medical prices including vaccination pricing
http://www.visamedicals.co.uk/us-price.htm
 
Medical questionnaire
https://uk.usembassy.gov/wp-content/uploads/sites/16/2017/05/USA-MEDICAL-QUESTIONNAIRE_IV.pdf

1) Shots- MMR, Tdap, flu. Your GP may or may not charge you.  
2) Medical summary printout, NOT a copy of your actual records. Also a separate short write up by your GP if you have had significant illnesses, conditions, or surgeries. Cancer, diabetes, asthma, high blood pressure, depression, etc, etc. 
3) Police certificate. If you have ever had any arrests, cautions, incidents, then also order a Subject Access Report which will detail specifics of charges, any fines, etc.
4) I-134 Affidavit of support from fiancé with 1 proof of income, 2 if you are nervous about it. London will approve on simply an employer letter for example. You don't have to show income five different ways to London.

Since you are close to getting the interview....and hopefully approved. My recommendation is to get SS# filed for as soon as you get stateside, before getting married. Then file shortly after for AOS. Life will be much simpler this way. AOS is usually anywhere between 3 months to maybe 2+ years.  EAD/AP is anywhere from 4- 6+ months if you  unlucky.

Hi, gather your police certificate and medical records from your GP (assuming you’re going through London). You should get your vaccinations done now to make it cheaper and easier.

The Acro police certificate took about a month to arrive when we were going through the NVC process so get that sorted ASAP. Also make sure you obtain your full medical record from your GP (not just the summary). try to get your vaccines form your GP (they are pricey at Knightsbridge). If you can’t get an appointment with your GP, Boots is a good place to go and get them done. Make sure your fiancé starts getting their stuff together for the affidavit of support (tax documents, employment letters, etc).

Up to 90 days but 2 - 3 weeks is typical.  With the holidays upcoming you may experience some additional delays 

They received my RFE response on 11/21.  Still waiting.

Yes you did if you arrived in the US and went through immigration. It's an arrival record, not something you "use" to enter such as a visa or VWP/ESTA.. 
 
Find it online. https://i94.cbp.dhs.gov/I94/#/home