I notice now several websites including UA offering US flights via LHR with EI/UA, not sure if this is a new phenomena or I've just become aware of it, certainly very competitive for EWR.
EI/UA have been code sharing for years, even when BFS-LHR operated this was available.
UA even offer cheaper fares from DUB to EWR via MAN/BHX/GLA/EDI instead of flying direct.