What difference between pickling spice and whole mixed allspice?

No it cannot, as they are both different things.

Mixed Spice it a blend of spices, (cinnamon, ginger, nutmeg, cloves, corriander seed, cassia, allspice).

Allspice comes from the dried, unripe berry of a pimento tree. It looks a little like a larger peppercorn... kinda.