Meanwhile back on planet earth.
Use your ATM card, it will work everywhere. Nationwide (apparently) charge no fees for overseas withdrawals, but in any case an ATM card will be cheaper than travellers cheques (do they still make these things!?).
It is probably wise to have say �50-100 of US$ on you for arrival, to get a taxi/bus, as even at JFK they only have 2 ATM machines, and one is usually out of order.
But changing money to US$ cash in the UK is uniformly a rip-off, compared to the rate you'll get on an ATM card...