Scandinavian Airlines has round-trip flights from Chicago (ORD) to London or Hamburg (LHR,HAM) for $449-$474 with a connection. London Flights in November, January, February, and March. Hamburg Flights in November and January. No Thanksgiving dates.
Some small OTAs on Skyscanner and Google have this for as low as $415; 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.
Carry on bag with a maximum weight of 18 lbs is allowed for free on SAS. You can avoid seat selection fees by accepting the randomly assigned seat at the gate. Upgrading to a higher fare class that includes a full-size carry-on bag for about $35 may be cheaper than booking the discount fare with the added carry-on bag fee (for about $70).
Click for more details and booking links Cheap Flights: Chicago to London or Hamburg $415-$449 r/t [dates vary] (No Thanksgiving) – Scandinavian Airlines