Merlin Parks aren't just able to invest money at will, without a business case, which is what they did in the early 2000's era.
Currently Merlin's interests are in Asia as that's where the emerging markets are.
If the British consumer, spent more money in the parks, and stopped complaining about the excellent value of the MAP looking for the cheapest options to the point of Merlin practically giving tickets away, share the fun, trade, promos etc, then perhaps they would invest more into the UK markets but currently Lego is the asset with the greatest return, hence why it will be getting a new land next year.
On the second to Disney comment, it's the truth however you have to look at the business as a whole for example the Midway clusters and Legoland parks generating millions in weekly revenue.