Courtesy of Atomicmpc
1. Login with your existing Steam account via the web and head to http://store.steampowered.com/app/550/?cc=US
(you can also do it via http://store.steampowered.com/app/550/?cc=uk)
NOTE: You CANNOT do this in the steam browser, it MUST be done in Internet explorer or Firefox etc. Its not a bad idea to close steam while you are doing this.

2. Add to cart L4D2 - ensure the link you click has the following URL - java script:addToCart( 2481); (NOTE: the cart number will be different if you use UK)

3. Purchase via credit card - it will state that your location is incorrect. (remember at this point to set your country to which ever site your using, whether its UK or US...but have the other details correct for billing address)

4. Change your country, state and zip to something local in the US - United States, CA, 90001 (considering in the previous step, I had selected US, the only states available at the time were in america, but one had the abrev. WA, so I used that one, with the 90001 postcode, it worked.)

5. Make purchase


Steam just validates location to allow the purchase for the regionalised (US/UK) copy. Credit cards just check the street address for authorisation. Thus they are two separate checks.

You can check this by matching the ID codes on the AU and US site

http://store.steampowered.com/app/550/?cc=AU
L4D2 Standalone ID: 2489
L4D2 Four Pack ID: 2490

http://store.steampowered.com/app/550/?cc=US
L4D2 Standalone ID: 2481
L4D2 Four Pack ID: 2483
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Many people use the UK one simply because its easier to distinguish whether your buying UK or not as the prices are stated in pounds. But I noticed the Au version has "Low Violence" under it.
Really though, as long as you make sure you are adding the correct item number you should be ok. (see list of Au & US cat items above)
**I take no responsibility if it doesnt work, I simply copied this method from another site and it worked for me.