You should study the guide.
1. You submit I-130, I-130a, fee, and all supporting documents to USCIS
2. USCIS will process the I-130 (about 11-12 months on average). Right now, there are more than a million I-130s sitting at USCIS waiting to be processed.
3. After USCIS approves the I-130, they will send the case to NVC.
4. You will then pay fees and submit documents (I-864, tax documents, income documents, DS-260, Police reports, etc.) to NVC.
5. NVC will process/approve document submission, then document qualify the case.
6. NVC will then place your case in a queue for your consulate (London) to wait for interview scheduling.
7. When your case reaches the front of the line, NVC will schedule the interview and send the case to the consulate.
8. Medical exam can then be done.
9. Foreign spouse is then interviewed at US consulate.
10. Upon approval of the Consulate Officer, the CR-1 visa will then be issued.
Average time, per other VJ members, is about 18 months from filing the I-130 to interview. You can see the timelines of other UK cases in the "timeline" area of V to see specific data. She can get medical and interview in UK.