Update: I couldn't find mixed spice on Friday, so I went ahead and made my own, figuring that I would find something else to do with the remaining mixture later. I was surprised at how readily the cookie dough formed in the food processor and how easy the dough was to work with ... as long as the surface is properly floured. My first few attempts, my cut-outs got distorted trying to pull them up. It is surprisingly hard to get a properly formed heart that way. (Yes, I know. Heart cutter = girly. Forgive me.)
Again, not sure what the desired size of these cookies was supposed to be - there's no "yield" notation on the recipe - but Nigella suggested about twenty minutes of bake time, and I had mine out after twelve. That's not even in the ballpark. I've noticed in general that her recipes tend to assume an oven that is much cooler than mine, so ... bake with care.
I used my leftover white chocolate icing to great effect here.
Final verdict (cookies): These are not as flavorful as I expected, but they have a subtle tang to them that could become addictive. Would try again.