I'd never leave a tip.
Most girls charge around £2 a minute. Next time you want to leave a tip, just think about that 5-10 minutes when she vanished off to hide the fist full of cash you've already paid her. That just cost you £10-20 to sit on a bed and wait.
Leave a tip once and they expect it the next time. If they don't get it, they may think about putting prices up.
Apart from the EE girls who only see a fraction of the money you pay, these girls earn a lot already in a 1 hour booking.
Maybe I'm wrong, but if a wg wants £100-120 for an hour, I expect her to be good. If she's charging £100 for a half hour I expect her to be bloody amazing.