No need to rush. They'll definitely let you stay and enjoy your meal for a generous amount of time!
In all likelihood you won't need much extra time, but if you want it, you certainly won't be rushed out the door.
In fact, most of the time the park closing time is (roughly) when they stop letting people into the parks, and they start encouraging guests to start heading toward the exits. Some of the stores will stay open a little longer, and the restaurants will continue as long as needed (within reason). Also, if you are in line for an attraction at closing time, they will usually let you stay in line and ride the attraction (some exceptions apply with Extra Magic Hours, and evening party events).
Hope that helps, and please say "Hi" to Pooh at the Crystal Palace for me!
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