They can't use a catapault. Cats are steam powered and the British carriers are gas turbine driven.
I don't know whether the British CVF's will have catapaults or not since the design is in flux. If they do, they won't be steam-driven as you have noted. They'll probably be electro-magnetic instead.
Okay, so they couldn't set up a small boiler for cat ops? I'm sure the steam powered carrier boilers are only bleeding off a relatively small amount for cat ops, so it's not like it would have to be a large boiler.
And I had also read that the Navy was working on new technologies that would replace our steam cats.
Still, for a carrier that is supposed to be close in size to a Nimitz class carrier, one would think that it would be designed for a faster operational tempo regarding aircraft launches and recovery, and based upon the artist rendering, it doesn't look like this carrier would suffice.