Mixed peel is what we call candied citrus peel. The mixed part refers to the use of peels from a variety of fruits. It provides your dish with the sweetness of the candied coating and the tartness of the citrus peel. It is indeed well loved in fruitcakes, but is also used in Christmas puddings.
Candied fruit is made by infusing the fruit with a mixture of sugar and corn syrup. Commercially made, it may also include preservatives.
In this country, it's easy to find candied orange, lemon, and citron peels, sometimes separate, sometimes mixed. Some packages also contain candied cherries, pineapple, and/or papaya, which are not citrus peels at all, but probably wouldn't harm what you're making.
Some diabolical British recipe writers use clever names like mixed peel to confuse us and keep us from making their specialties. You can buy their cookbooks – oh yes – but please don't try to make their fruitcake.