Universal isn't only coming to UK, it also unlocks the whole of Europe that has an easy boat, train or flight to a world class Universal park just like DLP. Europeans love their theme park visits when you look how much there is (for good and bad) and I'm positive lots of them will want to visit a Universal park without a +10 hour flight.
Regarding Potter, the stage show in London, the Studios and a theme park experience would only strengthen the pull to get visitors from further away. Again, don't just think UK but Europe.