The parks have multiple power suppliers, I guess to spread the load on the grid. The whole park isn't affected during a power cut. Making theme parks off the grid is not a viable idea and is of little benefit.
In regards to the 2019 TV ad, IMAScore made Thorpe a brilliant, brand recognisable theme tune in my opinion that is fully owned by them unlike Alton Towers and they don't even use it in the advert. wtf are they doing? 🤦♂️