I worry that it's your PC maker's choice of BIOS version and defaults here that created this issue. Here my current laptop let's me tap F9 (varies with model) and boot from USB. The UEFI BIOS is set to allow both legacy and UEFI booting.
Another BIOS choice (yours) is boot order. That is machine specific and not a Windows 8 thing.
I bought the Xtra-PC 64 meg because I wanted to try Ubuntu on my Lenovo Z580 which runs Windows 8 but there appears to be a block in Windows that prevents me from accessing the Bios Setup even from the command prompt >shutdown /r /0. What would be the the boot manager and boot loader code additions (or similar) for bcdedit so that I can boot to the USB Xtra-PC first and Windows 8 if the USB key is not inserted.