I’m sure it could have been a bird strike, but such engines are tested with all sizes of (grocery store prepared) chickens and turkeys to validate that any such strike won’t threaten the safety of a flight.
HF
Jet engines are fragile and a bird-strike can, and does, take out many aircraft every year (http://www.airsafe.com/birds/birdrisk.htm, http://www.airsafe.org/birds/BirdstrikeRates.pdf, for info on bird-strikes).
As a mishap investigator in the USAF, I worked one bird-strike incident where the jet was lost but the crew ejected. I also experienced a bird-strike and lost the engine.
Thing is, a bird-strike hitting the fan blades causes much damage and basically ‘unbalances” (if not destroy) the blades, and when those blades spin at over 22,000 RPM, the effect on the engine is like a shaggy dog shaking off water drops. . .pieces fly everywhere.
Which brings me to a question: If the winds at LHR were calm or even 10 kts tailwind, why would the pilot elect to fly over downtown London when the engine could very well spit parts (if not completely come apart), thereby spewing FOD all over the city and people below? Why not land at LGW and avoid over-flying a population center?
Anyway, jet recovered and no harm. . .other than to the bird(s).