How many people have actually damaged the glass they are working on? You'd have to be extremely careless/clumsy to do that. I can honestly say that in all my years nothing like that has happened to me. You need PL for things like people tripping over your hose, your ladder falling through someone's conservatory, dropping your squeegee on someone's head etc, etc.
I have the Master Tradesman policy from MMR (only available through brokers) it costs me about £70 a year for 1 million cover. I have only ever had to claim once, I stood on a radiator and it came off the wall
. Water ran down into the apartment below and caused a bit of a mess. The insurance paid up without batting an eyelid
There was no penalty the next year
Cheers,
Ian