Can anyone please help, I'm going to Florida next week and am looking for advice getting a USA Sim card for calls back to the UK, I'm wondering is it possible too go to a US T-mobile shop or stall and buy a Sim card then put it in my unlocked phone, would it be as easy as that?

Yes, you'd probably be better with AT&T, I think T-Mobile US uses a different 3G band, your phone might not be able to receive this band. You can get SIM cards at the airport.