You are reaching your "decisions" based on a lack of understanding of the basics of heating engineering.
You have apparently measured the open pipe flow rate whereas the relevant flow is the dynamic flow rate.
A combi boiler and distribution pipework will limit the actual flow rate because of their internal resistances. The reduction depends on the supply pressure and resistance and is usually about a half to two thirds.
If you hope to use more than one hot water outlet at a time then you need a hot water cylinder and a boiler of about 18 kW which will be able to modulate down to 4-5 kW and operate efficiently most of the time without resorting to turning on/off which is not efficient.
You should have two zones, TRVs and weather compensation.
Whilst I m not a big fan of Worcester boilers, their FP100 control unit is excellent and allows the setting of the proportion of in/out temperature measured to control the boiler flow temperature. Many others don't!
Tony
