Ships with country=usa will spawn as ground spawns and campaign mission targets. Having British ships with country=usa just gives you a bit more variety in your shipping formations.
If you change them back to Britain and you have a stock country.xml (which says that only usa will spawn for allies), you will not see British ships either as ground spawns or as campaign mission targets.
I often make up duplicate ships, one with british and one with usa nationality, in much the same way as AI aircraft are made, so as to get more spawning variety.