you don't need to go through any immigration or customs since you're on a domestic flight
The only thing you'll have to do is to collect your bags at baggage claim if you have any checked luggage/
Make sure in MCO that your luggage are tagged to newark and not to your final destination in the UK, since you're making a stop. But I guess you have 2 separate tickets.
You'll need to check bags again the next day and go through security (TSA) as it is for an international flight, check in starts 3 hours before the flight (at the airport, 24 hours for online check in), luggage must be checked at least 90 minutes before flight scheduled departure (they won't accept your bags if you come to the check in counter later than 90 minutes before flight) so be there early (3 hours is safe to beat the crowd)
You can be out of the airport in 20 to 30 minutes once you're off the plane.
It really depends on the time it takes for your checked luggage to come to the carousel.