Aren't the bookings subject to availability anyway? I recall having seen something in the T&C's that will cover them if they have to 'switch' aircraft or airline. Obviously with enough notice given.
From their website, note "operational reasons" which could be anything:
"That said, in some circumstances, we may have to change your seat. This would only be for operational, safety or security reasons. In accordance with the Civil Aviation Authority’s guidelines, certain requirements must be satisfied before we can allocate seats near the exit. If we do have to change your seat, we’ll try to keep it as close to what you booked as possible. A refund will only be offered if we can’t seat you in a similar position to your original request. A similar position means the same type of seat - window, aisle etc - as your original selected seats. In such cases, we ask that you keep a copy of your confirmation documents and boarding cards."