It would be genuinely interesting to know the mark-up on their offer. I have little doubt drinks are around the 200-300% range, with most of their convenience food not far off reaching the 400%+ range.
@Ash talks about places like Disney - the place where people are more than willing to pay £11 for a rice krispy treat, £5 for a small ice cream bar or £10+ for a bucket popcorn. The reason why people are happy to pay... quality and presentation. Yes, you do feel ripped off a tad, but you know you are getting something with a bit of quality behind it, or something unique to the park. It's not complicated - if they had standards for their food (not even needing to spend a huge amount more on raw materials) and added perceived value they would likely sell more without needing to lower the cost.
From a business point of view - I get why they outsourced. It literally puts all the issues and responsibility with staffing, procurement and legal stuff in the hands of someone else. I do think, however, it is crazy that a company the size of Merlin don't have their own wholly-owned catering subsidiary. You would think that from a cost-to-profit ratio this would make sense, and allow more specialist operations.