can a UK macbook be charged in USA with 100v charger?

Yes, the chargers will work anywhere. All you need is a plug adapter.

MacBooks have auto-switching to accommodate the voltages of various power supplies throughout the world: http://support.apple.com/kb/HT1981. Also, you can always use a charger of a higher wattage to charge a mac that normally uses a charger or a lower wattage, but not vice versa.