回复：Really?! What should I do? I have scheduled the "appoitment"

I think the appointment you scheduled is probably for the finger print. Even though the finger print work is at a USCIS office, you will need to schedule the appointment through the website of Bristish consulate. Please take a closer look.
Once he goes for the finger print, the appointment notice will be stamped with USCIS. This stamped paper will need to be included in the visa application package you send to the British Consulate.