I can definitely confirm that replacing your old main which terminates in 15 mm
copper
with 25mm MDPE wil very substantially increase your flow rate whatever the
pressure is.
I recently replaced my old steel incoming pipe which was about the same ID as
15mm copper, and old and internally corroded, with 25mm MDPE and the two ran
"side
by side" for a while, with valves to allow the house to take via the old or the
new.

The max flow rate into the bath with both bath taps running, filling a bucket,
was 13 litres a minute on the old incoming pipe and a massive 45 litres a
minute on the new ! !
I fact it was more, as a fair amount was spraying around the bucket the force /
flow
rate was so good! - I had also replumbed everywhere with 22mm copper as far as
possible
so pressure drop was minimised in the house - anyway, shower now gave a flow
rate of
some 20 litres a minute which was too much ( on a meter now) so fitted the
smaller bore
shower hose which dropped it a bit to some 15 litres a minute, which is still
good!
If you heat water on demand (combi) I would say you also need to consider
whether the boiler
will cope with two showers running ? Mine was a 105,000 BTU , and I like
cooler, high flow
showers - flow reduces in winter and / or with hotter showers.

Go for a 25 or 32 mm re-plumb and keep the bore big in the house as far as
possible.
The water company may charge more for larger pipe sizes !

Nick