LHR would probably be a non starter in the real world. I was thinking about a situation where it could be feasible.
Your destination is Las Palmas (two long separate runways), in the middle of the night. ACE and FUE are both closed after 11PM (if memory serves). So usually, TFS would be the obvious choice. I'm ignoring TFN for the sake of the argument. Just as you check in, TFS closes due to a large hole in the runway, stranded aircraft etc, so it is unavailable. LPA is CAVOK and traffic is light until the morning.
It would not be unreasonable to dispatch given the above, although I would take the fuel for TFS anyway and keep updated on the LPA conditions on the way down.