How can I get child support from my ex who moved to the UK?

Because your ex-husband has moved to London, your situation involves international law. There is a specific set of rules called the "2007 Convention" [cite:ca-divorce-act-sec28]. This is the "Convention on the International Recovery of Child Support and Other Forms of Family Maintenance" [cite:ca-divorce-act-sec28]. This agreement is designed to help a "creditor"—which is the term for a former spouse to whom support is owed [cite:ca-divorce-act-sec28]—obtain payments from a "debtor"—the former spouse who owes the support [cite:ca-divorce-act-sec28]—who lives in a different country. To make these rules work for you, you will likely need to work with a "Central Authority" [cite:ca-divorce-act-sec28]. A Central Authority is "any person or entity designated... that is responsible for carrying out the duties that are imposed under the 2007 Convention" [cite:ca-divorce-act-sec28]. This entity acts as a bridge between the legal systems of your location and the United Kingdom. When trying to understand how these international rules are applied to your specific case, you can look to the "Explanatory Report on the Convention on the International Recovery of Child Support and Other Forms of Family Maintenance" [cite:ca-divorce-act-sec28p2]. This report is used to help interpret how the 2007 Convention should be used in practice [cite:ca-divorce-act-sec28p2]. It is currently unclear how a court will determine the exact amount of unpaid maintenance or the new monthly amount based on your original agreement without further legal proceedings. State, provincial, or local laws in California may add further rules regarding how to initiate these domestic claims before they are sent internationally; you should check California state family law guidelines.