Skyscanner tend to deal with lo-cost airlines only, but the other sites posted can be useful as they scan many operators at once. A1Travel website is very good for charter flights, it also used to tell you which tour operator it was but now they dont seem to anymore (perhaps too many people were using it as a research tool and not booking!)
Another way is to check the airports website. They all give details of who is flying to where, usually listed under Charter or Holiday Destinations. The only problem is these lists are not always exhaustive, they often are absed on premliminary plans, and by the time the summer comes these may have changed. According to the respective airports websites, operating in May to Orlando are:
Birmingham
Thomsonfly (Britannia) - Thursdays for Thomson Group (Thomson, Skytours, Portland Direct) and MyTravel (Airtours, Direct Holidays)
Thomsonfly (Britannia) - Fridays for Thomson Group (Thomson, Skytours, Portland Direct) and Thomas Cook Group (Thomas Cook, jmc, Sunset)
East Midlands
First Choice Airways - Tuesdays for First Choice Group
Manchester
Air Atlanta - Mondays and Thursdays for Thomas Cook Group*
MyTravel Airways - Tuesdays and Saturdays for MyTravel Group
Monarch - Thursdays for Cosmos and Avro
First Choice Airways - Fridays for First Choice Group
Thomsonfly - Fridays and Saturdays for Thomson Group
Thomas Cook Airlines - Saturday for Thomas Cook Group
Like I said, these are not always 100% accurate. For example, Travel City Direct operate an extensive programme from Manchester to orlando with Air Atlanta, which isn't listed. Then again, I do wonder why Thomas Cook would be using Air Atlanta, maybe thats an error and should read Travel City? Also a quick search threw a Monarch flight on a Monday - not listed on the Manchester site.
Unfortunately, due to the nature of the charter market I don't think you'll find a site with a difinitive uop to date list, but a good search of a couple of the ones here should be fine.