I'm doing a house renovation, and the hot&cold water feeds to the
showers in the bathroom and ensuite are in 15mm copper. I want to
upgrade to 22mm where possible to get better flow, as the hot water is
from a cylinder with header tank.

My view is that I'd rather install 22mm everwhere where it won't be
easy to replace later, like underneath tiled floors, and behind tiled
walls. There will still be some runs in 15mm (or maybe half inch, as
some of the pipework is very old), but I might replace these
eventually.

Is this a reasonable thing to do? Are there any reasons why I
shouldn't install 22mm where possible (apart from cost, which is not a
huge factor when looking at the whole job)?