Thanks for testing this Neil!
From my testing on both Android and iOS tablets of the current AT I found that both double taps and drag and drop don't work at all. To be fair they are only used on a few screens so if we could find a quick way around this then it shouldn't be too hard to implement.
I can appreciate what you mean about buttons getting in the workflow. I feel like before design is started on the design refresh (we are currently doing a quicker UI update). We should research how to UX that allow tablets use without creating a slower interface on other platforms.If we come to the conclusion that double taps are indeed better then maybe we could enable 'long-presses' instead of double clicks.