Link: http://www.icelandexpress.com/booking/book_your_flight/
Travelling to London and back starting Sept 20 through Dec 13 is $445 round
trip including taxes.
The cost of the flight from EWR to LON: $220.50 (flights are Monday/Thursday
)
The cost of the flight from LON to EWR: $224.50 (flights are Monday/Friday)
The airport is Gatewick.
The next cheapest I saw on kayak was $663. Hope this helps someone!