Checking on ITA, the genuine taxes for LHR-ORD/ORD-LHR non stop each way are GBP284 - taking that off of the GBP1900 comes to GBP1616
Assuming fare same both ways and that it is a single sector each way, that would make the inbound fare GBP808
So would work seem to work out at GBP606 for the downgrade