namante

I'm trying to book tickets for my trip (june 1st - 30th) and I was wondering if anyone with experience could help me out.

My itinerary starts in London and ends in Italy but since prices for flying from Italy to JFK (NY) seem a LOT higher I dont mind taking cheep flight back up to leave from London. Heres what I was looking at:
Round trip JFK to LHR - $634 from STA Travel on Air India
or
One way JFK to LHR - $231 from STA Travel on Air India
One way LHR to JFK - £209 (~$380) from STA on American Airlines (you cant book 1 way trips from UK to US on the US website, only the UK website)

Just to save the hastle of booking the two tickets seperatly I was just going to buy the round trip, but for only $540 i could do this:
One way JFK to LHR - $231 from STA Travel on Air India (same flight as round trip)
One way DUB to LHR to JFK - $309 from Expedia on BMI (to LHR) and then American Airlines (to JFK)

So why is it cheeper to fly from Dublin to London to NY than from London to NY? The cheepest flight one way I can find from LHR to JFK is $550+. Is it possible for me to book the ticket from DUB to LHR to JFK but just show up at LHR and catch the 2nd leg of my flight?