Danny, with age and wisdom has distilled the argument into 3 succinct sentences.
And the question of numbers Tornados is a red herring. Militarily this is a low intensity operation that could last a decade. Politically Cameron needs to get stuck in so that he can say to other countries "Follow me".
Tornados are the visible photogenic front of that political will whereas Reaper is probably cheaper, more economical, and from a psychological perspective a better weapon.
It can sit over a target like a Damoclean sword, visible or not as needed.