Hi Eli, I just flew back from the states to england, having gone through the whole body scan and pat down.
I was flying with a female passport, wearing binder packer all that jazz, absolutely no problem.
Of course you'll pass as male, so a man just pats you down, literally just feel for weapons etc, it wasn't uncomfortable at all.
I would say however, don't wear a reelmagik if it has a flex rod, as this will show up on the scanner and be confusing, whereas one without just won't show. As I walked past I checked out my body scan out of interest, and it's completely androgynous anyway, it's not like some of the scary pictures I'd seen where the person literally looks like they're stood their naked. It's just a human outline with any metal highlighted. They get you through so quick anyway, they're not paying attention to things that to us would flag as trans, they're looking for stuff that to them flags a danger.
Basically, I wouldn't stress it at all, I was quite worried about it, but the experience was completely fine. You'll pass great by then and they'll just treat you as any other member of the public. I don't even remember him patting down my crotch to be honest, so packer or not, providing it has no metal, I'm sure it'll be no problem. Have a good trip!