If UA have ticketed you allowing that time, they will get you to your destination; even on another airline.

However I am surprised that they have allowed such a short time. You can be held up for a long time at Immigration and then you have to clear customs and re-check your baggage.

On the bright side, they know at the gate for the Orlando flight who is arriving on the London plane and if there are enough passengers on that flight, they will often 'hold' the Orlando flight.