I have exactly the same problem, an Aussie living in the US and I just posted about it this morning. It seems there's no way to side-step the locking of language to location-services, which to me is simply bizarre and extremely short-sighted.
The language choices elsewhere on the iPhone do not seem to imply or restrict any localtion-related services, the location of your carrier or Apple account or sometimes GPS location is determining. Not making Siri work this way is just inviting frustration for no good reason as far as I can see.
Anyway, probably the only way to get Apple to fix this is to get as many people as possible filing bug reports on the feedback link that Tom posted earlier in this thread. Time for some crowdsourcing.
Meanwhile, I'm practicing rolling my R's and pronouncing every vowel completely and with a nice twang.
Me too I'm having the exact same problem, im a Brit in LA. I changed siris accent to British English so that words like WATER ( aka US 'wadder') etc could be understood and it worked perfectly, now I get told to change it back to US English if I'm searching g for businesses in LA.
Seems to me as though Siris accents are connected to their country of origin?
I think this is something APPLE are going to have to work on. I did hear someone say that Siri over time learns your accent like txt used to work but so far not much luck for me.