Dell Venue 5050 USB Boot

We are trying to image these DELL Venue tablets but there is no option to boot to USB. 

The only option we have is the Windows Boot Manager but it just goes into Windows, it gives no option to boot to the USB drive.

Yes, I have a tried pressing F12 and yes I have gone into the BIOS to see if there are any boot options, there is only one; Windows Boot Manager. Once you select this option after pressing F12, it proceeds to loading Windows.

I have considered disabling the secure boot option but I am a bit reluctant as it stated I may have to reinstall the OS and the only way I would be able to do that is from a USB device.