I would have thought that the limiting factor was the pipe run from the
main
to the stopcock and if this has a bore of somewhere near 15mm it doesn't
matter what I do in the house. Am I wrong?

Only flow rate is really important, provided the pressure is enough for a
good shower. However, there is a reasonable correlation between pressure and
flow rate, as it is the pressure that makes the water move. There are plenty
of exceptions, though.

Provided you are happy with the flow rate provided by the cold mains, and
realise that opening a hot tap might mean that the same flow is shared
between the taps (it might not, of course), then you can use 15mm for the
hot water, too.

There is some benefit to using 22mm within the house even when 15mm is used
outside. You must not think of the system as a "weakest link" effect. Many
people do and find the system odd. It is actually more akin to an electrical
network with resistors. The 22mm has less resistance than 15mm, so having a
system half 22mm and half 15mm is better than a system all 15mm. It doesn't
matter which order the pipework is in.

However, in a hot water system, there is a disadvantage to using 22mm in
that there is more cold water sitting in the pipes to clear, which means a
longer wait for hot water and less efficiency when all the hot water in the
pipe sits around cooling off. This disadvantage doesn't apply to cold water
mains, though.

The ideal is to use 25mm+ MDPE incoming main and then 22mm throughout for
cold, preferably from a manifold system to reduce side effects. The hot is
then run through 15mm, provided flow rate is good, or 22mm if required to
reduce pressure losses leading to slow flow rates (very unlikely to be
required on an instantaneous combi, due to low achievable flow rates).

Christian.