Everyone always say that one of the biggest flaws in the UK theme park scene is that it is just a monopoly and that is true given how that's caused the stagnation of the Merlin parks, however ironically when you think it, it had been their own fault of resting on their laurels for so long that it has by accident led for places like Paultons and Drayton to begin a comeback in which they should be in a place nowhere near Merlin however given how far the latter have lowered the bar, it has only encouraged the other parks to rise to that bar.
Hell, you'd be forgiven to think that Merlin committed suicide with their monopoly by giving their parks a chance and given how they'll likely be, on current projections, be in a far solid place come 2030, Merlin will have no one to blame but themselves for possibly being knocked off their perch and that's nit including the Universal juggernaut hanging overhead.
Merlin does not have a monopoly on the UK theme park industry, not has it ever had. It has been the largest operator, but falls massively short of having a monopoly, or even behaving like a monopolist.
A monopolist has so much power that they control the industry's price of the product, the servicing and production contracts around the product, and the consumer has no other choice.
Merlin doesn't have industry control over pricing. Entry prices have broadly stayed consistent for the past 20ish years of operations, increasing just under the inflation rate. Their product has got cheaper, not more expensive. Merlin's pricing also doesn't affect the pricing strategy of their competitors, with Paulton's, Blackpool and Drayton regularly being more expensive to visit than a Merlin park.
Merlin does not have exclusive contracts with suppliers, nor do they control how much that contract is worth. Other parks in the UK can, and do, go to the same attraction providers as Merlin and pay the same price as everyone else. Merlin does not have purchasing power to bully their suppliers.
A monopoly means that you don't have another choice, there aren't any other parks one can visit. This isn't the same as not having any other parks of a similar and consistent high production quality.
Merlin isn't the only seller and there are close substitutes.
A monopoly ensures that there are high barriers to entry. Either the costs of starting up are too high, or government intervention makes it impossible, or the essential resources are controlled by the monopoly. Whilst there hasn't been a new park, of considerable note or size, opening in the UK during Merlin's existence, there have been expanding existing players and a new one attempting to enter the market, with Universal.
Lack of innovation is perhaps the one usual telltale monopoly sign which ought to be considered, but even that doesn't hold up to much scrutiny. The level of Merlin investment in their parks fell off a cliff edge when they went public, which is typical for a company that's just floated. The Smiler crash ended up being a financial drain, caution was exercised and significant capital went on the payouts and defence cases. Merlin going private again, provided a corporate shakeup and signs of investment. A worldwide pandemic, shaking the entire entertainment and leisure industry out a hold on investments too. Merlin aren't not investing because they're a monopolist, they haven't been investing because at a corporate level they've been distracted and a mess for about 10 years.
Merlin isn't a monopoly. It's a overgrown entity which is starting to struggle from growing too quickly, at the expense of consistency and quality.