Choose your arrival and departure airports after deciding which Tests to attend and which cities form the route. The same airport does not suit every itinerary, and assuming London as both arrival and departure point is not the right default for every traveller.
An open-jaw itinerary — arriving in one city or airport and departing from another — may reduce unnecessary backtracking for some multi-Test trips. If the series ends in London but the trip begins elsewhere, a return from a London airport may be more practical than retracing the route. Whether an open-jaw works for a specific itinerary depends on the full routing and cost comparison, not on the general principle.
Compare the full itinerary rather than only the headline airfare. A cheaper outbound fare that creates a longer or less practical arrival journey may add more friction than the saving is worth, particularly if it affects arrival margin before the first intended day of play.
Arrival should leave enough margin before the first intended day of play. Arriving on the morning of day one and expecting to reach the ground in time for the start is a tighter arrangement than most multi-leg itineraries can reliably support, particularly with luggage and hotel check-in to handle.
Departure should not be booked too close to the end of the final match attended. Test match final days can end at any point; booking a departure flight that requires leaving immediately after the last over is a risk that affects the whole trip, not only the final day.