As mentioned you can in this case choose from various UK airports - the cheapest flights are available when departing from London Gatwick, East Midlands, Glasgow or Birmingham. The earliest flights are currently available at the end of May 2015. Your connection flight possibilities from your home city is fastest to check through Skyscanner.
Booking samples
London - Florida for £249
Birmingham - Florida for £279
East Midlands - Florida for £279
Glasgow - Florida for £269
Flights are operated by Thomson Boeing 787 Dreamliner. Please note that only hand luggage up to 5kg is included in base fare, you would have to add £70 if you would require check in luggage up to 20kgs. Remember to arrange US visa or ESTA certification prior your outbound flight to USA.
Booking sample of a cheap flights form UK to Florida, Sanford for £249 here. In this case you would be departing from Manchester to USA on 18th of April. You would be then returning back from America back to UK on 4th of May 2015.