most often, its one of there things for aircraft like that:
Aircraft released early from maintenance, this can happen more often than people realise.
Aircraft out of service for maintenance, not in maintenance yet, short flight possible.
Aircraft scheduled with long wait (not uncommon for 747s and other long haulers)
Aircraft had previous maintenance issue, now resolved, but its own flight was substituted.
weather caused several flight cancellations, aircraft being redeployed to cover seat needs.
It could also be done by a knock on: ie Aircraft A is assigned to do roundtrip that will take 4-5 hours. Aircraft B is running 20 mins early so takes Aircraft As flight, Aircraft C takes Bs, and so on until Aircraft A can take a flight out of the origination point again.
Most likely for LHR
, its crew training/service testing.
Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety.