For the UK spouse of a USC resident in the UK, it can vary between nothing to report to a full disclosure of all income and assets. Two questions are of importance: first, how does the USC report to the IRS, married filing separate or married filing joint, and second, are there any joint accounts?
Married filing separate with no joint accounts equates to no reporting for the UKC. Married filing seperate with joint accounts equates to the USC reporting the joint accounts, possibly to both the IRS and FinCEN. Married filing joint equates to the UKC spouse electing to be considered a US Person for IRS and Treasury purposes with a full reporting obligation of all worldwide income and assets, for every subsequent year (unless the election is revoked, a once in a lifetime option).
Xpost with Kelly85