Seems like a very imprecise term, doesn't it? In this day and age we like clear definitions and knowing what's what. And we have seen the term mixed spice used in many ways: "Indian salt with mixed spice," "mixed or pickling spices," combinations of nutmeg, ginger, clove, and cinnamon; of ground coriander, cumin, turmeric, saffron, fenugreek, and/or curry powder; of ground cumin, dried oregano, and chili powder; whole or ground, dried or fresh.

In you case, though, there is some agreement on the British mixed spice or, rarely these days, "pudding spice." As mentioned elsewhere at Ochef, it is a mixture of allspice, cinnamon, clove, coriander, ginger, and nutmeg (listed alphabetically, not in order of importance). The quick-and-dirty substitution is pumpkin pie spice, but that almost certainly leaves out the clove and the coriander (which plays a pretty prominent role in the mixture).

You may actually stumble upon a "mixed spice" mix, though – we are constantly surprised at the number (and often quality) of the spice companies and their wares that are hidden in the woodwork in this country. The fact that most of them can't get shelf space in your local supermarket doesn't mean that their products can't be found in natural foods stores, ethnic markets, etc. An online search, though, turns up just about any combination of spices – and really nothing that you want. Mixed spice is definitely not a trendy blend, so just make your own:

16 parts cinnamon 8 parts coriander 4 parts allspice 2 parts ginger 2 parts nutmeg 1 part ground clove

…that is:

Mixed Spice

4 tsp cinnamon 2 tsp coriander 1 tsp allspice 1/2 tsp nutmeg 1/2 tsp ginger 1/4 tsp cloves

(all ground), which will yield about 2-1/2 tablespoons of mixed spice.