Help! U.S. spouse applying for UK visa

Hi, I'm British and my wife is American. I am in the UK, but she has to apply for a spousal visa from the US, and it seems she has to do it via VFS Global. However, we urgently need to get her back in the UK and really want to get the premium/priority processing time, which as I understand is about 6 weeks for a partnership visa.

I've read online that the 'premium' slots are renewed at midnight, but even browsing the VACs in several US cities, there don't seem to be any.

She called VFS and spoke to a woman who said both that she didn't know anything, and also that my wife can buy the premium service at the VAC when she attends her biometric appointment.

Is this actually true?? Some people online say it isn't. And VFS are so unhelpful.
We cannot wait for an appointment that might never offer her a priority/expedited processing time.

Can anyone please help?