I'm about to fit a thermostatic shower, running it off the bath supplies. I was going to run 15mm pipes up the wall to the shower with isolation valves just after the pick off under the bath. But just having got the valves I see the internal size of them is only 8mm. Is this going to cause a problem with the flow rate? The supplies are from the cold tank in the loft and the hot water cylinder, with a good flow to the bath taps.