US flights abroad are actually working with the UK and other destinations to accommodate ESA. According to the United page,
https://www.united.com/web/en-US/con...e_animals.aspx
"United is approved under the United Kingdom's Pet Travel Scheme (PETS) to accept service animals on flights to the UK. Review the entry requirements on the gov.uk website. Please provide a minimum one-week advance notification. Additional fees may apply."