There were some good deals doing MAN-LHR-MIA but I'm not sure whether that covered Easter periods. MAN-PHL-MCO or DUB-PHL-MCO can be good value but you need to take into account getting to Manchester or Dublin.

I'd keep checking the options from all London airports to Orlando, Miami, Fort Lauderdale and Tampa. Possibly set a Google flights alert. Easter is one of the busiest times to go to Orlando but as a semi reference point I managed to bag 3 WT+ returns LGW-MCO from 22nd Dec to 31st Dec (so another busy period) for £900pp which was cheaper than economy.

Edit: I also agree with the post above about trying to book as a car and flight package as the savings can sometimes be huge.