Chopsie,
It all depends on the work you do. If you clean domestic houses only and none of these have conservatories then you have a point. If you clean commercial premises, schools, shop windows or car dealerships then the excess of £250 could be negligible compared to the cost of replacing a window pane. For a start, they will want it doing by the company they use and then the costs are out of your control.
Also, most standard policies exclude cover for cleaning gutters, frames, soffits, fascias etc. They just cover you for cleaning windows, thats it.