Yep, major parks completely from scratch are few and far between, and the rate of success is also far from consistent.
You can burn through millions just getting the core infrastructure in place like utilities, IT and roadways. Remember, this isn’t just a case of slapping down some rides and theming. There’ll be massive undertaking, laying car parks, getting power and plumbing to the site, building amenities like toilets and catering, not to mention all the back office requirements for admin duties, distribution centres/stock management warehouses and workshops.
For some perspective, DCA originally cost $600M back in 2001, followed by $1.1B in 2007 to try and actually put the place right! Disney SEA was around £2.4B based on current exchange rates but not accounting for inflation. Both of those were second gates, so there was already some degree of infrastructure in place. More will need to have been added no doubt, but it wouldn’t be a completely blank slate. It would probably also be fair to assume that the cost of construction will be greater in the UK/Europe than elsewhere in the world, due to different processes and regulations as well as importing and transporting resources into the country.