I managed to install it and so far seems activated with the key I purchased after doing the following. I guess I'll post steps I took in case anyone else has trouble.
Get the Windows 8.0 Pro ISO Build 9200 (MD5 hash 1CE53AD5F60419CF04A715CF3233F247E48BEEC4).
Extract the ISO to a temporary directory.
Create a new text file "ei.cfg" without quotes.
Put the following into the new text file you made
Save the text file.
Put the text file into the "sources" folder inside the extracted ISO folder.
That's it! Now you can turn the extracted ISO folder back into an ISO with a third party utility, and burn it as your "Windows 8 OEM" install disc.
You can also run setup from the extracted ISO folder and install Windows 8 this way.
Nice thing is it won't bother you about entering a product key until it's installed and booted, and you just need to select the correct version ("Windows 8" or "Windows 8 Pro") for your OEM license for the installer.
When Windows 8 boots it will ask for a product key. You can select skip (it's kind of hiding on the screen) to use the command line instead. I skipped and entered my product key with the command line.
I did the following to activate:
Run command prompt as administrator (hover mouse to bottom-right corner of desktop, click "Search", type cmd, right-click cmd result and click towards bottom "run as administrator).
Type the following into the command prompt you just opened (where xxxxx is your product key license)
slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
And Windows should have successfully accepted your OEM product key license. So far my UK OEM Windows 8 Pro license key seems to be activated this way.