I'm sure you'll be ok to query them, no matter where you're from. However, bear in mind that it'll be easier working with an agent in your home country; and that you'll pay a higher commission to your agent on overseas sales, so sales to the USA will be at that higher rate. As you'll probably make more sales in the USA than in the UK it probably makes more sense for you to be represented by an American agent, if that's where you live.