To what extent does a soldier in American/British forces customize their own gear??

Question

..out of their own pay packet??

Answer 1

I don't know about the the Brits but US Soldiers can do very little to customize their gear! They have to use the guns, ammo, uniforms, communication equipment, vehicles, etc that they are issued.  

Answer 2

Can't speak for Brits, but for American's it used to be fairly minimal and consisted of minor departures from issue. I recall a lot of Gortex items before being adopted, customized knives, alternative ruck sacks, off market sights when luminous sight posts were installed, helmet pads because Kevlars suck. That sort of thing. Nothing really major.