Dismissing Big Sur permission dialogs via keyboard
I had no intention of answering my own question, but when I was prompted again this morning on starting another vagrant instance, I just decided to have another go at pumping key combinations in the hope of hitting one I haven't yet tried.
Found it!
Usually in dialogs the Tab key is used to navigate between elements. Obviously tried that, tried the various arrow keys, with and without modifiers. Then I hit on the combination that, for whatever reason, I hadn't tried before.
To move between the two options, as there are only two in this specific type of dialog, the combination is ...
Shift+Tab
I probably hadn't tried that as usually Shift is only used to reverse direction. When there are only two elements, just hitting Tab should be enough to toggle.
Anyway, I assume the extra modifier is required as this is a security permission dialog so you are forced to do just that little bit more work.
In the scheme of things, this only saves maybe seconds per day, but I was just finding it really annoying. Hope this helps anyone else.