The previous answer is perfectlly true; as a UK citizen you will have no problems entering the USA with a Cuban stamp in your passport. having said this, on my recent trips to Cuba my passport has not been stamped. However, a word of warning. The importation into the USA of many Cuban products is prohibited; therefore, if travelling back from Cuba with cigars, Cuban rum etc via the USA and a third country, I expect that the Cuban items would be confiscated.