I, too, think that you'll piss them off! I wouldn't be surprised if they shared the numbers of time wasters among colleagues: you could find yourself blocked by quite a few girls.
You might try to negotiate a discount for a very long booking, but doing it for a 30 minute booking will be very hard.
Yes, getting paid £50 instead of £60 is still better than being paid zero, but many wgs may not want to work for below the quoted rate. Plus, if a wg agrees and word gets spread around, she'll never be able to charge full rate.
I wonder if any girl offers any kind of loyalty discount (eg every 5 punts, the 6th is half price) but I guess that's unlikely. Many threads on saafe on how they (understandably) hate it when a regular asks for discounts.