What can I do?

Hi There, I have recently replaced my shower system with a thermostatic valve. I have a gas bolier in the kitchen about 10 years old. When I turn the shower on it's eithe rtoo hot or too cold. The boiler will only fire up when I turn the hot water tap on full. As soon as I turn the tap to balance the temperature with a little cold water, the boiler switches off and the water is cold again. The plumber has told me that, to shower I will need to have the central heating switched on while I shower and switched off when finished. Is this correct? Is there no way we can fix this issue. Grateful if you can shine a ligh ton this..