I always fly Premium Economy because I am a wheelchair user and getting into a PE seat is much easier than a regular seat.
At Gatwick you have a separate check in (last time it was the same as First Class), so we had a much shorter line.
Most times, I have been sent to the Express line at passport control, so I have walked straight through to the departure lounge, though I don't know if this was because I was PE or because I was in a wheelchair.
If the plane is very full, a few lucky passengers will get bumped up from economy to PE to make room. PE passengers will get bumped to first.
On my last flight to & from Orlando, my wife and I were sitting in First Class seats downstairs (PE was upstairs on this flight and I can't climb stairs). They had some dual purpose seats at the front of the economy cabin, just behind regular First Class which can be either overspill seating for First, or PE. You still only get economy food.
Andrew