is this a real question? Anyone can work anywhere as long as you have the correct paper work and or visas.. and you don’t need a license to work in an architecture office since you need experience before getting one anyways.  You just need to be better than everyone else availble. 

To work in USA, you will need to have an employer who is willing to sponsor your work visa. There are costs associated with preparing those documents, and some firms choose not to hire foreign workers to avoid these costs. It doesn't mean it's impossible, but it's a tough process.

You could first find work at a firm that has offices in both UK and US and try transferring internally. That's how I know some people did it. Those companies also do this kind of paperwork all the time, so no problem there, maybe it's a write-off...