In the latter part of the last decade, Disney World was more-or-less content with what it had and became flaky in regards to building additional large-scale, high-quality attractions, the same way Merlin seems to be acting in Britain now. Then, down the road in Universal Orlando, Harry Potter opened and upped the theme park game in Orlando. That strong local competition basically forced Disney World to respond first by expanding Fantasyland in Magic Kingdom and then form plans to build a whole Avatar section in Animal Kingdom (and of course a new land in Hollywood Studios about a certain big-name scif-fi film franchise).
That's the key to getting Merlin off their butts: competition. Either bring in one of the other international park chains to establish a presence in Britain, who will compete with Merlin on their level, or unite a handful of the large non-Merlin parks in Britain and form a new British-based chain of parks. That kind of competition is something that Merlin would not be able to ignore.