Home water pressure/flow rate

I'm looking to replace my boiler and have already asked for suggestions on this forum, but I have a question about my current setup.

I have a Mira Excel shower. It has a thermostatic valve and mixes the water to ensure a constant temperature and it gives a good pressure providing no other water source is turned on in the house.

As soon as a hot tap or a cold tap is turned on the shower pressure drops in the shower as it tries to maintain a constant temperature, which it does eventually, but usually after a short burst of hot or cold water.

I can understand why turning on a hot tap would affect the pressure; the boiler isn't capable of producing enough hot water to feed the shower and the tap simultaneously, but why does turning on a cold tap affect the shower?

I believe our flow rate has been measured at 20l/min and the pressure is "very good", whatever that is! We live North of Leeds in Wetherby at the bottom of a hill (which I've been told helps the pressure).

I'm assuming you have a combi boiler set up? If so you are totally dependant on your water mains flow rate. If you think about it if the source to the boiler that is your cold mains is reduced the boiler will not be able to process enough hot water thus reducing your flow. don't mix up pressure and flow rate.

Yes, we have an old combi boiler, but it does have a 50l internal hot storage (it's a Saunier Duval Thelia Twin 28e). There are no header tanks on cylinders it is all closed so we rely on the boiler for heating and hot water and there's no backup.

I've determined that the cold water supply coming into the house is reduced to 3 bar by a pressure reducing valve. However, the outside tap connects to the mains supply prior to the pressure reducing valve so this is at mains pressure. If I measure the flow rate from the outside tap it is about 20l/min, but from the kitchen tap it is approx. 12l/min for cold and 10l/min for hot. This was crudely done using a 1 litre jug and a stop watch!

This suggests that the pressure reducing valve is also reducing the flow rate!

I understand the relationship between pressure and flow rate (I think), however, in the house I guess the hot water and cold water are both at the same pressure (3 bar) and because the pipes are all 15mm, the flow rate should also be the same, although the flow of cold water is slightly better than hot.

I guess this therefore explains why even turning on a cold tap affects the shower as the flow of water is not sufficient to keep the shower and another tap going at full blast!

If we had an unvented cylinder fitted and a system boiler, would the cold water supply around the house still need to be reduced? If so, I we would still have the same problem in that any taps turned on would affect the shower? But then I thought unvented cylinders were supposed to resolve this type of issue providing the flow rate into the property was high enough! If we didn't have to reduce the pressure of the cold water supply then I would expect that turning on a tap would NOT affect the shower as there would be sufficient cold water to feed both the tap and shower simultaneously. So, what is needed is to increase the flow rate of the hot water. However, I seem to recall reading that even unvented cylinders must have the cold water feed reduced to 3 bar, so I'm now totally confused!