As Norman states, there won't be any really low fares in July. Additionally there aren't that many scheduled airlines which fly direct from the UK to Florida.
According to the Orlando International Airport website, the only flights from the UK are with BA (from London) or Virgin Atlantic (from Glasgow, Manchester or London):
http://www.orlandoairports.net/goaa/marketing/ intl.htm
The Miami International Airport website doesn't list destinations but the most likely airlines to offer flights from the UK are probably American, Continental, BA, United and Virgin Atlantic:
http://www.miami-airport.com/html/airlines.htm l
The cheapest flights to Florida are usually into Sanford. The only scheduled operator is FlyGlobespan (from only Glasgow or Belfast) but there are plenty of charter operators, several of which will offer flight-only options. For a full list, see the bottom of this page:
http://www.orlandosanfordairport.com/destinati ons.asp
Examples of charter fares currently offered (including taxes and charges):
MyTravel (ex Gatwick 03/07, back 17/07): �404.40
http://www.mytravel.com/AniteNextPage.asp?p=SP ECIFICPROMOTION_59771&s=462220459
Thomsonfly (ex Gatwick 06/07, back 20/07): �563.98
http://www.thomsonfly.com/en/index.html
Monarch (ex Gatwick 07/07, back 21/07: �565
http://www.monarchcharter.com/index.htm?domain =mo