If the system shipped from Dell with Windows the key should be present.  Not all systems do -- some ship with Linux, or without an OS if the original order was corporate and shipped without Windows.  Check using the service tag at dell.com/support -- the original OS will be listed on the manifest.

If PC was originally shipped from Dell with Win 10, the key is in the bios but it is an OEM key locked to the specific pc motherboard.  it would not be useful for another pc.  Whereas if some one bought a retail license Win 10 key, it can be used on another PC.

• Type “cmd” into the search bar.
• Right-click on the Command Prompt and select “Run as administrator” 

• copy and paste the following command: 
  wmic path softwareLicensingService get OA3xOriginalProductKey

another fine detail.  the oem key if present is for a specific edition of Windows 10 (home vs pro).  when you do a clean install of Windows 10, the latest MS installation media automatically looks for the embedded oem key in bios and installs the same edition but newer version of Windows 10 for you.

Thanks for the replies guys!

The current nvme has Ubuntu installed on it but it would have definitely shipped with Windows I would have imagined. There's a Windows sticker on the back. I'll try a fresh install later 😉