What is your preference?

a) Spend two extra days in Montpellier on BA's expense.
b) Return home at approximately the original time.

If a), then don't question BA's statement that this was the earliest flight back and send BA the bill after the flight.

If b), then try to get BA to book you on another airline or from another airport and send BA the bill for rebooking yourself if BA refuses to rebook you. If you can rebook yourself for less than the original cost, then rebook yourself and request a refund of the cancelled flight.