A few possible answers.
1. Given that UA is a USA airline, it is more well-known in the US than in the UK. This may allow it to charge a premium for US-originating flights vs. UK-originating ones.
2. People originate from EWR because it is a more convenient airport for them (i.e., people who live in New Jersey). People originate from JFK because it is a more convenient airport for them (i.e., people who live on Long Island). These preferences allows UA to charge a premium for US-originated flights. In contrast, people coming to the U.S. from the UK typically don't care whether they arrive at EWR or JFK. They are just going to take public transportation into NYC.