Hey JL,
Are you trying to sit the written before 1st August by any chance?
If so I had the same idea. Called the FAA at Oaklohama City to confirm what are the requirements as I hold an EASA ATPL. They told me I needed a verification letter to proceed. Takes a minimum of two weeks to process on their side. Plus the UK CAA are pretty slow to process stuff. I've had to accept that I'm out of time and I'll have to sit the damn ground school course in the future!
Hope you have better luck!