Hello, I was in this same position. Our NOA2 was January 18, 2024 and we became "Case Ready" on NVC website on February 16, 2024. We have also been waiting on the Welcome Packet. On https://uk.usembassy.gov/visas/fiancee-2/applying-for-the-visa/ there is a specific line that states what to do in our position, "If more than 2 weeks have passed since your petition has been approved and you have not received a letter from the Embassy, please contact us through the contact form."
As a result, on February 29, 2024 we filled out the contact form (https://uk.usembassy.gov/visas/iv-contact_form/). When you fill it out make sure to include your NVC case number, NOA2 date, and NVC Case Ready date.
On March 1, 2024 we got a response stating, "Embassy records confirm that a letter outlining visa application instructions was mailed. Based on the information you have provided, it appears that the letter did not arrive. You need not be concerned because this email can be used in place of the letter. Therefore, we will not be mailing a replacement letter." You will want to print this email for your embassy interview. The rest of the email (and also https://uk.usembassy.gov/visas/fiancee-2/applying-for-the-visa/) explains the next steps (which I've also listed below). (preparing documents, DS-160 (make sure you print the confirmation page), sign up for medical examination, attend medical examination, sign up for courier, sign up for embassy interview (must be AT LEAST 5 days after the medical examination), attend embassy interview).
Hope this helps