Queen Cox is a self-fertile sport (or possibly self-fertile seedling) of Cox's Orange Pippin. It has a slightly improved coloration, and the flavour is just as good as the original.
Anecdotal evidence suggests that it is a bit more reliable than Cox's Orange Pippin - though this is still a very challenging variety to grow.
There appear to be two forms of Cox going under the name Queen Cox, although they may be related. The first form, which is referenced at the UK National Fruit Collection, comes from a natural bud-sport of Cox's Orange Pippin which was found in an orchard in Berkshire, England, in the 1950s. This form is not self-fertile.
The second form was developed at the Long Ashton research station near Bristol, England, in the 1970s and sometimes known as Cox SF18 - presumably the 18th seedling in the development programme The focus of this development was to raise an improved self-fertile Cox - and this form is indeed self-fertile. It is possible that the Long Ashton development used scion material derived from the Berkshire form.