Clearly there are a lot of factors that affect how many visitors a theme park gets, but that sounds quite poor to me. You’ve got a lot of Merlin employees in the area, between Thorpe, Chessington, Legoland and the London cluster, all using their Magic Passes to get friends and family in for free. You’ve got a lot of school groups with big discounts. You’ve got all the Merlin passholders who are potentially getting heavily discounted entry, depending on how many times they use their pass.
When you consider how much they must have spent on intellectual properties and big roller coasters, it doesn’t sound like a great return.
No one ever pays full price to get into a Merlin attraction, pretty much ever. There are always discounted routes in. Booking on line, newspaper vouchers, cereal and soap promotions, the list goes on.
You do have a cluster of Merlin employees in that area, but how many of them do you think want to hang out at work, or visit, on their days off? There may be a small amount, but not 1.5 million of them.
What you do have, however, is the third largest metropolitan area in Europe, with over 14 million inhabitants (2016 numbers) in London alone.
Visitors are visitors, whether they've paid that day the full sticker price or not. Where Thorpe does make it's money is on all of the extra spending in the park. The merch, the food, the drinks, the photos, the fast track, the games, the arcades, the parking, the list goes on.
Cinemas don't make profit on you buying a ticket to a film, nearly all of that goes to the distributors. The make a profit on everything else they upsell you on.
They do have quite incredible returns, just not in the obvious places.