Friendly beautician done my eyebrow waxing and tinting as expected. Good price in central London. Happy and positive ambient in the saloon. I would come back.
So this is how Splash works: there is a bunch of hairdressers waiting for clients to go in, usually, young guys, just seating everywhere. As soon as you enter and say you have a booking with Treatwell, the receptionist will go and speak with your hairdresser. S/he will explain to your hairdresser that you booked through Treatwell and that your service is already paid for, basically pointing out to your haridresser that there will be no tips. From there, the service is obviously less friendly, as they know you already pre-paid for it. They will also try to do it as fast as they can, so they can move into other clients who (hopefully) will pay in-store and get tips from them.
In a nutshell, if you go to any Splash (I've been in three of them) and you pre-paid through Treatwell, you will be treated differently because they know you have paid already and they won't get tips from you.