To be honest I think Merlin are pretty short sighted with their attitude towards dark rides. If Towers or Thorpe were to install a ride as well themed as Vallhala, it could really pay off. Vallhala is now in its 13th season and, even though it costs alot to run, the public love it and I include teenage and young adult thrill seekers in that.
In the UK, I think Drayton Manor's dark rides are often overlooked. They aren't up to international standards but rides like Pirate Adventure are pretty decent but sadly aren't what Drayton Manor is famous for, probably why so many other UK parks are sceptical about installing them