$353 on select dates from BWI and IAD to LGW with a less than 2 hour layover in Iceland. Some dates are even cheaper ($270) but have overnight layovers, which if you want to see Iceland for a day, might be worth it.
Last day is December 17th 2018
Kids are cheaper, for example 1 adult and 3 kids add up to $1050.
Click the "Dates" option to see other dates.