If I have a preinstalled copy of Windows 8 on a computer and I downgrade to Windows 7, can I use the Windows 8 licence key on another machine?
I know I must use a third party software to get the licence key (which seems odd as I should have access from within Windows 8).

No, the Windows 8 license that came preinstalled on your computer is tied to it forever. So yes, you can downgrade to Windows 7 if you have a license you can use, but you can't transfer preloaded Windows 8 license else where.

OEM versions of Windows 8 are identical to Full License Retail versions except for the following:
- OEM versions do not offer any free Microsoft direct support from Microsoft support personnel
- OEM licenses are tied to the very first computer you install and activate it on
- OEM versions allow all hardware upgrades except for an upgrade to a different model motherboard
- OEM versions cannot be used to directly upgrade from an older Windows operating system