Good question! There are a few routes in my data where the outbound and inbound sectors don't match for this exact reason. Since almost all of our flights begin and end at LHR, if a flight is cancelled we either operate the flight the following day, or fet "positioned" (our word for deadhead) home as a passenger.

Usually when a route changes aircraft, there is a requirement to "position" some pilots out a few days before as passengers to bring the aircraft home when it lands there for the first time. Logistically, very complex!