Of course BBC, ITV and Sky would be planning for EU elections. They knew well in advance the possibility that this could happen, no one could be immune from the coverage since January.
Behind the scenes all news companies plan for every eventuality, BBC, ITV, Sky will be no different. I doubt there will be the large scale studio sets used at the general elections, more likely it would be smaller presentation studios.
At the same time, with Brexit hanging over it, from what I've noted on politics forums, is that many (on both sides) are considering this almost like a second referendum - Mentioning the possibility that this could be the largest turnout for this kind of election in this countries history, assuming it goes ahead of course.
Will broadcasters take note if this is the case and give it a large set?