Hi Jason,
I agree with Dave. Transforming the data in post-processing is a more accurate option. Heights in ETRS89 are ellipsoidal. So you can use them directly. It’ll allow you to get drone coordinates and heights in ETRS89 as well. And then, you can transform them to OSGB36 + ODN heights in the photogrammetric software.
If you have only orthometric heights, you can follow the steps from this post to set them on the Reach RS2+ base. In this case, Emlid Flow will recalculate the heights to ellipsoidal.