Have just run a test. I replaced the shower head on the thermostatic mixer shower in the bathroom with a gauge and turned on to full. I checked the readings over 20 minutes: between 1.75 and 2.2 bar.
Then I turned the cold on full in the bath. This caused a sharp but gradual drop on the gauge (still attached to the shower hose) right down to 0.1 bar. Other taps cause a lesser drop to around 0.8 - 1 bar.
Would changing the incoming supply pipe really make such a difference, especially when factoring in that most of the pipework within the property is 1/2" or 15mm anyway?
And, bearing in mind that drop to 0.1 bar, perhaps I would be better off with the Mira which claims to perform better at low pressures?
I know there's a lot of marketing guff out there, but Mira say:
"Mira Magni-flo™ technology means you can have a drenching shower even with water pressure as low as 0.1 bar."