Icelandair has $428 round trip tickets from New York JFK or Newark EWR to London Gatwick LGW at $428 round trip for travel departures January to March. Google Flights shows $369 and $390 fares, but I did not find any fares that low. The low fare is $159 outbound, but $259 inbound for $428 total is lowest fare I find at time of writing. This fare is available from both JFK and EWR.
You can add a stopover, up to 7 nights in Iceland. for about $15 more. Many of these $428 low fare dates force an overnight in Reykjavik, Iceland on the return. I selected two examples below that do not include overnight stays in Iceland. Winter nights are a good time for catching the Aurora Borealis in Iceland.
Icelandair New York JFK – Reykjavik KEF – London LGW – KEF – JFK
Icelandair Newark EWR – Reykjavik KEF – London LGW – KEF – Newark EWR