The agreement was probably made and exists under US law, specifically the state in which you made the agreement. Enforcement can be sought in a UK court which will apply US law to it.
However, it is highly likely that your "agreement with a friend" is not a contract and will not be legally enforceable because it lacks one of the fundamental requirements of a contract: intention to create a legal relationship.
As detailed in What is a contract and what is required for them to be valid? one of the presumptions the court will make is that an agreement of a family, domestic, social or voluntary nature is not intended to legally bind the participants. You would need evidence to overcome that presumption: a document stating this and signed by both of you would be best. Without this, if you go to court you will probably lose. Sorry.
Either way it looks like you will lose the friendship.