Icelandair has round-trip flights from Chicago (ORD) to London, United Kingdom (LGW/LHR) for $506-$515 with a connection. Flights October to April. Extended weekends available. Origin/Destination reversible.
Some small OTAs on Skyscanner and Google have this for as low as $457; which is cheaper than the airline. If you plan on booking there, beware of any site that charges a fee to cancel a ticket within the 24-hour “free” period. Use unknown booking sites at your own risk.
A Carry on bag (20-lbs @ 21.6 x 15.7 x 7.8 inches) and a personal item (15.7 x 11.8 x 5.9 inches) is allowed for free on Icelandair. You can avoid seat selection fees by accepting the randomly assigned seat at the gate.
Icelandair offers free stopovers, which good if you want spend a few nights in Reykjavik. More details are on their website (link).