Game Pass availability on PC Australia

I don't think you read your error correctly

Gamepass is available in your region but xcloud isn't which is what you are trying to do

You can use any xgu pc titles but you can't gamestreaming in your region xbox titles to your pc

According to the Xbox Game Pass for PC website:

The Xbox Game Pass PC Games plan is currently available in the following markets:

Argentina, Australia, Austria, Belgium, Brazil, Canada, Chile, Colombia, Czech Rep, Denmark, Finland, France, Germany, Greece, Hong Kong, Hungary, India, Ireland, Israel, Italy, Mexico, Netherlands, New Zealand, Norway, Poland, Portugal,
*Russia, Saudi Arabia, Singapore, Slovakia, South Africa, Spain, Sweden, Switzerland, Taiwan, United Arab Emirates, UK, US

*(purchase at your local retailer)

It can be found in the FAQ section under "Is Xbox Game Pass for PC available in my market?"

Unfortunately since Game Pass for PC is not available in your region, it is not possible to purchase it.

The Xbox Game Pass PC Games plan is currently available in the following markets:

Argentina, Australia, Austria, Belgium, Brazil, Canada, Chile, Colombia, Czech Rep, Denmark, Finland, France, Germany, Greece, Hong Kong, Hungary, India, Ireland, Israel, Italy, Mexico, Netherlands, New
Zealand, Norway, Poland, Portugal, *Russia, Saudi Arabia, Singapore, Slovakia, South Africa, Spain, Sweden, Switzerland, Taiwan, Turkey, United Arab Emirates, UK, US

Ultimate is just a bundled subscription of Gold, Game Pass for console, and Game Pass for PC. It doesn't change what games are available on Game Pass. Doom Eternal is available with Game Pass for PC.

My console keeps trying to purchase in the UK?