Virgin Atlantic has just announced a one-way NYC-to-London fare of $189 based on a round-trip purchase. The fare is good for flights departing from Kennedy Airport or Newark Liberty Airport, and require a Saturday night stay.
Taxes and fees of up to $250 (!) aren't included, so that hikes up the fare quite a bit, unfortunately. Still it's worth factoring it into your comparison shopping, as other airlines will have similar add-ons. The nice thing about Virgin is that accompanying children ages 2 to 11 pay only 80 percent of the fare. The downside is that children under 2 without a seat are charged 10 percent, so there's no free ride for junior.
Details and booking are here.