You could try turning your mixer to full hot, and using the domestic hot water thermostat on the combi boiler to set the temperature. With the boiler in the kitchen, though, an assistant is useful in this phase
If that makes things worse, not better, then you could try replacing your shower mixer with a thermostatic one -- about £120, but it will be worth it and will fix your shower woes (if not your shower hose and shower rose) for good. A thermostatic mixer continually adjusts the proportions of hot and cold water so even if the pressure or temperature vary, it will always try to maintain the same temperature at the outlet.
The spacing between the inlets on modern shower mixers is standardised, so it should be a simple enough replacement job -- turn off the main stop valve (with a flow heater or combi boiler, this will also cut of the hot water, as this valve also supplies the water heater), unscrew the union nuts from the old mixer, replace the seals (buy new ones from the same plumber's merchant as you got the thermostatic mixer, if there isn't a pair in the box), offer up the new mixer and tighten the unions. While the water is off, take the opportunity to install a screwdriver-release isolating valve in each of the feeds to the shower, so if you ever need to work on it in future you can turn off the water just to the shower without affecting anything else.
Electric showers: A typical combi boiler puts out about 30 kW of heat, and can reduce the power almost to nothing. An electric shower heater puts out about 10 kW (if you have a 45 amp breaker) or 7 kW (if you have only a 30 amp breaker), and always delivers that amount of heat to the water -- you adjust the temperature by adjusting the flow rate; faster flowing water undergoes a lower temperature rise. It takes 70 watts to make water flowing at one litre per minute, one degree hotter. Suffice it to say that with a combi boiler and a thermostatic mixer, you will be able to enjoy a hotter shower at a better flow rate than an electric shower.