Try travelocity.co.uk and enter LON to NYC (giving you all airports in both dests.)
Swiss had the lowest fare but wouldn't confirm the test dates I tried.
Ba.com won't go lower than 554 ex LHR or MAN to BOS or JFK
Aer Lingus (aerlingus.com) quotes LHR-SNN-BOS-SNN-LHR out 16.7 back 23.7 for 500.85 stg. (and Shannon has US immigration preclearance) - you could try looking for LHR-JFK too but you might have to move back/forward days to get the best fare.
[edited - had a look at JFK, out 14th back 26th 450GBP]