Not wishing to teach a Granny to suck eggs, Eliza but if you are with an agency and shilly shally prevaricate when they offer you work, you may well find you do not get asked again., I agree it is a very good way to get a feel for a school and that can decide you to apply for a vacancy should one arise, OR never to darken their doors again, But you have to get that foot in the door first, so I would do my reseaarch well in advance. In my day however you would have been unlikely to be offered a job as the agency would be entitled to a fee for placing you so it would make sense to approach schools independently, setting out what experience you have, what your subject is and whether you can teach any other subjects with reasonable confidence. That way the school is under no obligation to the agency. Of course on the other hand, if a supply teacher is needed at short notice a school may well not ring round but go direct to the agency. It's a tricky position.
Last weekend, in Rutland, the first statue in Britain of the late Elizabeth II was unveiled.