My understanding is that leucism causes a lack of pigmentation, whereas this bird appears to show an excess of pigment in some areas, and a lack of it in others. Perhaps partial leucism & melanism then, though melanism tends to cause the whole plumage to darken?
Actually Dave, seeing your images, which are somewhat clearer and sharper than those of the OP, this puts me more in mind of a Tree Sparrow I saw a few years ago, that showed similar wear in the breast. It made the dark bib appear much more extensive, rather like a Spanish Sparrow.
It turned out that it was feather damage, most probably caused by a mite infestation or similar parasite. Contour feathers seem to be affected worse than the underlying down feathers.
I don't think it's that uncommon, and some species are more prone to it than others.
IMO it's more like a semi-albino. Albinism is a lack of pigmentation while leucism is "bleaching" of pigments - dark colours look much paler / washed out. There is a semi-albino Oystercatcher on the North Wirral at the moment that has a patchy appearance like the photo ( except it's a bit more extensive ).
Most certainly a Coal Tit - the colouring makes me think that it cold be dietary. Blackbirds and corvids fed on an incorrect diet can appear - in the late1950's some individuals hand reared blackbirds and corvids on specific diet in order to create "mutations" that could be sold!
There is a strain of mutation Coal Tits, technically Lutinos, being bred in Europe - and alao Opal Great Tits - whilst I do not like mutations or the breeding of them, I could make an exception for the Opal Great Tit!