Impedance Matching: Distance H is to prepreg or to core ?
The height is the distance between the trace and the ground plane.

If you have a signal on the top layer, and a ground plane after the prepreg layer, it will be the thickness of the prepreg layer.

If the signal is on one side of the core, and ground plane on the other side of the core, then it would be the thickness of the core.

If you have a signal on one layer, and then the ground plane two layers away (e.g. Sig|prepreg|core|plane), then it will be the combined thickness of both.



As a side note, 2116 is an FR4 prepreg material. According to slide 6 of this presentation, it has a dielectric constant of roughly 3.6-3.8, although it seems to vary depending on the number of layers. For two layers, the next slide shows it goes up to about 3.9-4.3 which is about the same as an FR4 core. 

I had a quick look, and several PCB companies I found use two layers of 2116 to build up the required thickness. So you can probably work on the theory of it being similar to the FR4 core. However in practice if impedance is that critical, you would make test pieces to measure and characterise the impedance.