Without having read any of the Unreal Engine movement code, there's a complication here, and that is that "jump" might be influenced by the floor that the player is standing on. The force impulse might be related to the slope in some way. You have to figure out what "double jump" means in the context of there being no floor.

Placing a floor that has the influences you want under the player is, in a sense, a hack, but not an entirely misguided one. A better design might be not relying on whatever the "jump" button does, but writing your own, e.g. applying an impulse to the player directly.

If this sounds like semantics, consider a game like Super Mario Galaxy, which has spherical worlds. How do you determine the correct impulse for which way a "jump" goes? This is where these sorts of complications come from.