Re: U.K. submissions to the U.S.(Young Adult)
The most obvious question is whether young novel will work in the US market. Have you read anything recently published in the US that has similarities?
Agent first is certainly the best route. Many US publishers will not look at unagented submissions, and those that do take a geological era to reply.
I like agentquery.com as a reliable resource for US agents (beware there are many scam agents in the US, particularly one called the Children's Literary Agency)