I think you should brace yourself for the possibility that this might not work. £1500 is still cheap for a transatlantic J to Florida, this is bank holiday weekend so it is therefore quite likely that for that budget, you'll have to make do with premium economy (or possibly one way business and one way straight economy). As Tobias-UK says, it would be useful to know what is the best you have found so far so that people see if they can improve it further, but quite frankly, a month before your travel dates, prices are more likely to go up than down.