Adding a data point:
BWI to LHR first week of January (before the snowstorm hit): WTP to CW for $789 (appx £600) offered upon request during check in.
Would have accepted if travelling on my own but the two of us ended up enjoying seats 10A and B (front row of the WTP cabin on the 788).
Good seats (had to call the Silver line when OLCI opened to move myself into A (bassinet seat) for both in an outbound).