The UK Sailors since they are on the allied side (US/UK) will work @ 50% ability.

They will recieve normal EXP for their level vs ship level.

I suggest not doing so and getting some actual US sailors there. Or if your trying to level UK sailors, level them on UK ships. There is no advantage to other nations sailors on another nations ship.

i do this only when i want certain sailors to get to higher level faster (ex.  if you want to have higher level gunners on the 2nd nation in order to perform better)

other than the exp gain, i see no reason to put sailors from other nations on your monty

When using allied nation's sailors on a ship only experts work not vets.