To be fair, I didn’t ever suggest that the 1.5 million visitors were all Merlin employees and their friends. Clearly they’re not.
However, I suspect a lot of them do hang out at the parks on their days off. Most of these staff are young people and Merlin attractions tend to have a lot of churn in their staff, particularly at the seasonal parks. It’s not like these are mostly people who’ve worked for Merlin for years and grown tired of the parks. For a lot of the staff, the free tickets are a big perk of the job.
It is probably worth adding as well that the thing about nearly all the cinema ticket going to the distributor is a bit of an urban myth. It's not completely untrue. There may be certain films where for a short time it is largely true. Often films work on a ratcheting system where the fee goes down the longer the film's been out. Also, some films are more likely to sell more drink and food depending on the target demographic. But by and large, it isn't the case that most of the money goes to the distributor. If it were, then all cinemas would charge the same price, but you can get one cinema charging £3 to see a film and another cinema on the same day charging £20 to see the same film. If almost all the money from ticket sales went to the distributor, there wouldn't be any advantage in a cinema charging more than the minimum price.
It may be true that for certain big films that lends themselves to lots of food and
drink sales, most of the ticket sale will go to the distributor, but that would still only be for the first few days after the film's released. There are big margins on cinema food, but often not enough on their own to sustain the costs of running a cinema.