I think you misunderstand. Surface Pro supports only UEFI, not legacy BIOS. What you are referring to as a "BIOS interface" is in fact the UEFI firmware settings. If you're having issues accessing the settings, this article may be helpful:
https://support.microsoft.com/en-ca/help/4023531/surface-usi...
I'm not sure what you mean by "install your own copy of windows" but you can certainly boot from USB by following the instructions here:
In one case I’m familiar with, they were getting pummeled over poor O365 user experience in 2015 because the customer was refusing to upgrade past Office 2007.
Everything they’ve done in the last 5 years is about pushing out software faster and making traditional enterprise IT too expensive.