Missed approach at LHR, low fuel, land at STN perhaps?
Also, some ADS-B equipment doesn't seem to distinguish between a 7600 squawk and a 7700 squawk from what I've been told. Don't know if it's a player in this case.
I would think Flightradar24 and FlightAware would have good coverage in this area but perhaps there are so many targets that the odd track is a sewn together plot of intermittent ADS-B reports:
Delta (DL) #30 ? 05-May-2015 ? KATL - EGLL / LHR ? FlightAware