I'd be surprised if anyone else even bothered doing this (Don't mean to sound rude)
A lot of effort and risk purchasing from the USA.
Why didn't you just buy a second hand one from the UK?
Or just upgrade to a new HU?
Just my 2cents, but US is left hand drive, could that also mean the plugs plug in to the buttons the other way round than a UK one? So basically plug it in back to front. Even that I don't think it will work.