Answer:
If the UK phone in the USA has roaming enabled, and most will, then you dial the standard UK number.
The cellphone system will find the phone, no matter where it is in the world.
Be considerate when dialling phones that are out of the country. The person you are calling pays excessive charges for the incoming call.
Roaming is being used by evil cellphone companies to generate enormous profits. They charge extremely high tariffs for contacting roaming phones, and the person who is roaming pays for the international call as well as the normal call rates paid by the person who is calling.