There is space if we move the water filter out of the way, which is fairly easy to do.
I was also thinking, if we think its the mains pipe that could be the issue, can we temporarily put another one from the meter to the stopcock (run it through the house). Then we can see how everything behaves?
This is what I am thinking!
step 1, disconnect mains and connect new mains to outside, do dynamic pressures and flow rate with open outlet
step 2, reconnect mains with new pipe from meter (if accessible) to house
step 3, do dynamic pressures on mains with system connected
step 4, check every single outlet (that is accessible) for dynamic pressure, flow before and after taps.
step 5, buy one tap and replace in several places to see if this makes a difference
step 6, breaker tank and pump
Obviously if it any of the above fixes it then we know the issue.
I believe it is the mains for these reasons:
The flow may be good but the pressure can be bad, due to reduced pipework size.
For instance if you put your hand over a hose pipe, you think the water is coming out really fast (flow or L/min), however if you were to measure it, it is actually really low (Bar/ Dynamic pressure)
So a blockage could be giving a good flow rate but the pressure is low.
This in turn would hit the taps and the taps would then restrict it even more.
For example if you were to run a hose and twist it a little in the middle, you will still get a reasonable flow, however if you were to kink it twice it would barely dribble out.
Hope this explains it well!! someone might be able to explain it better.
Just out of curiosity, did you fall out with the builder at any point?