The most important thing to consider is not whether the piece includes Bb or not, but which note is serving as the tonic, or home tone.. This is the note that, simply put, sounds like the end of the song. It has a very stable sound, and it gives us a feeling of rest or conclusion. Wouldn't you agree that your last note, F, gives us that feeling that the melody is done?
Now, try a little experiment. Replace your last note with a C. Does that note give you a feeling of rest and conclusion, or does it give you a feeling that the song is unfinished? Were you to play it for 100 listeners, 99 would likely say that the note F sounds final and the note C does not. For this reason -- our perception of finality -- F is the tonic for this song.
To be correct, we should add the B-flat to the key signature to show that F is in fact the tonic (even if there are no B's in the song).
Why is F the tonic? Why can't it be C? There are two reasons that stick out. First, we are very used to the interval of the perfect fourth (here, C to F) working as 5^ 1^, or sol do for solfege readers. C-F tends to happen a lot in F major as 5-1, and much less frequently in C major as 1-4. Another reason suggested by Mixnik is that the notes of the F major triad appear on many strong beats, and this tends to set up F as the tonic, too.
If you wanted to write this in the key of C major, slide every note up 5 steps. Your opening sixth would be G-E, and all those parts that used to outline the F triad would now outline the C major triad. Your final note would be C, and it would really and truly sound like the tonic.