All instant water heating appliances suffer from this. Our instant shower is usually set around the four or five mark on the temperature gauge in the summer time, but it is now being set to either six or seven. We also notice the flow dropping when we set the gauge to a higher temperature setting, because that's how these things work. You slow down the flow rate to keep the water in the boiler (heater tank) a bit longer so the heating element (exchanger) has time to heat it up to the temperature you need.
You could fit a hot water storage tank dedicated to the shower and fed from a loop off the central heating system.