On the x86 platform, there is the option of turning off this regulated boot process which is being called, rather fatuously, secure boot. This came about due to opposition from other companies. But on the ARM platform, secure boot is mandated by Microsoft and there are no exceptions. You pays your money, but you have no choice.
Ubuntu’s plan involves having a key which is distribution-specific in the firmware, and also a key vouched for by Microsoft. CDs which are sold or distributed separately from hardware will need Microsoft’s key to be present in the firmware to boot while bootloader images from the Canonical website will have Ubuntu’s own key.