I've been using Google Music since before most Americans even. My cousin on the Android team invited me within minutes of them announcing it at I/O
Are you not able to sign up from
http://music.google.com ? If not, then yeah, you could get someone in the US to try.
Also, the version of the Android Music app that has Google Music enabled, may not yet be in the UK Android Market. If you google for an app called MarketEnabler, then provided you have root, you can spoof the location and carrier using that app, so that you can load different regional versions of the Market. So if you use that and set it to spoof a US carrier, the app will show up. That's what I had to do on the first day.
I have over 10k songs in my Google Music account now