If you're flying directly back from Belfast to Newark on the UA nonstop your flight won't be leaving from BHD (Belfast City Airport) which is where you'll arrive on the EI flight from LHR. Rather your return from Belfast to Newark on UA will leave from BFS, which is the Belfast International Airport near Antrim which is about 15-20 miles outside of Belfast. Just be sure to give yourself enough time to get out there by taxi or otherwise, as it'll take you at least a half hour (traffic permitting) or longer to get there from downtown Belfast.
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