Hi there,
I'm going to assume you want to burn miles with the upgrade so my advice is slightly tailored to that!
An easy way to check is to use
http://www.rewardseats.com and select the route from LGW to MCO and search for the dates that you are flying to see if there are any reward seats available. The site has had a bit of a upgrade so it will now present M&M and Reward and Cash fares so it helps with your task!
http://www.rewardseats.com/rewards/vs/Actually purchasing the upgrade gets a bit complicated, as you need to check what class of ticket you have purchased and then check to see if there is reward seat availability in Upper (known as a G class).
the-toolbox/fare-codesUpgrade path is as follows to go from PE to Upper:
K > S or W > G
You buy a K class ticket, pay the fare difference to get to a S or W and then use miles to upgrade to the G in Upper. You always need to buy into the fare class that gives you the option to use miles for a upgrade
Sometimes its just cheaper to buy a S or a W and sometimes its cheaper to buy a Z class (lowest priced Upper ticket) but its all a matter of testing the different options to get the best cost or milage use.
Have fun!