I always work on the principal that if a pilot wants a visual, he/she will ask for it. Probably cos years ago I was taught that you shouldn't entice a pilot to make a visual approach because you didn't know what his view of the weather conditions were and it could lead a less experienced pilot to accept an approach which may not be the best one at the time.
Anyway, at a unit like Bristol many of the crews are based or at least regulars and you get to know who's going to ask for a visual.