When replacing the stop taps, use full bore lever-operated ball valves
instead of bib taps.

In a later post, you ask whether check valves are essential. Where
does the water come from for the shower - is it gravity flow from a
header tank for cold and a vented stored hot water system - or is it
all at mains pressure, with an unvented hot water system or combi
boiler? You only need double check valves if there's a danger that
dirty water could get sucked into the mains in the event of a mains
failure.

Mains - and mains feom Combi. I suppose syphoning could remotely be
possible if shower head was left in bath tub water.

Copper pipe - 30 years old so should by 15mm. Not keen on solder in loft.