Yes, self employment income can be part of the Foreign Earned Income Exclusion.

For self employment, is her income covered under the UK Social Security System?  You would need to real the Totalization Agreement, and *IF* it is not subject to US Social Security, she would need to get a "certificate of coverage" from the UK for you to include on the US tax return.

https://www.ssa.gov/international/agreements_overview.html

https://www.irs.gov/government-entities/federal-state-local-governments/totalization-agreements

https://www.irs.gov/pub/irs-utl/2019ntf-30.pdf#page=17

Don't forget about things like FBAR, FATCA, and if she is working as an 'entity' (LLC, etc.), which all adds more forms.