US dollars. You'll have no problem exchanging currency and cashing travellers cheques in Florida - both can be done at a wide variety of locations. Dollars can be withdrawn from ATMs for a small charge.
English, but large percentages of Florida's population speak Spanish, particularly in Miami
Most UK citizens do not need a visa for holiday to Florida that's under 90 days, but a visa waiver form should be completed upon entry to the USA, and you must apply for an ESTA before you fly.
Click here for more information on the ESTA