If you have a North American account, I think all you need to do is go to your WoW directory and open up reamlist.wtf in Notepad and then replace whatever is there with "set realmlist us.logon.worldofwarcraft.com" without the quotes.
I believe the EU client should say "set realmlist eu.logon.worldofwarcraft.com".
I tried to do that in reverse to play with a Danish friend and that editing worked for contacting the EU servers, I just didn't have an account to play EU WoW.
It should work in reverse though, since you have a US account. The only problem will be that since it's the EU version you'll probably have an unnecessary vowel at the end of every word and you'll probably have to choose what language.
Edit: Also, you'll probably have to download the UK updates too. I don't know though.