I have no idea whether this is possible, but as a USN permanently residing in the UK, I wonder whether CBP might conduct the interview for you at the Consulate in London now that GE is available to UK Nationals.

It is at least worth a call to either Citizens Services or directly to CBP's attache at the Embassy. It's entirely possible that they may not have thought about it.

Barring that, all I can suggest is the walk-in, even at a station which says it doesn't do walk-ins. Maybe your story gets a result.