Alfie, the system is set up to provide designed flow rate for a specified size of boiler if the system set up is as specified in the manual.
You need to be more precise with your question as your setting of half flow at the tap makes no sense. What setting is the HW thermostat at? What flow rate have you got at the tap (remember half open tap for you might be a trickle for me when my tap is half open)? What is the cold water temperature? Is your boiler set up correctly? is the burner pressure correct? Is the flow rate as specified in the manual? TOO MANY questions to give you an answer.
All that can be said is, after the cold water has been purged, regardless of what the flow rate is upto the maximum rate of delivery specified for that boiler, HW thermostat determines what temperature is delivered at the tap maximum being 35 degrees plus the temperature of cold water at book value flow rate for that boiler.