This also applies to the common noun sandwich, the food item named after the Earl of Sandwich. I actually did a BrE preference survey for this word, which revealed that 53% of my sample preferred -wɪdʒ in this word, 47% -wɪtʃ. (Some people, apparently, have tʃ in the singular noun but dʒ in the plural and the verb.) Actually, sandwich is unusual in that the historical w is not lost — unlike Norwich, Greenwich, etc., which have no spoken w in the suffix (in BrE).
There are other exceptions, too. Ipswich ˈɪpswɪtʃ retains w and has no variant with dʒ. Similarly Nantwich, Middlewich, Droitwich, and Bloxwich. You may notice something about the syllable weight of the first part of the name here.
Apparently Colwich, Staffs., the location of a serious rail crash in 1986, is ˈkɒlwɪtʃ, despite the light first syllable.
Strangely enough, the same hesitation between -ɪtʃ and -ɪdʒ is found in the common noun spinach. The OED tells us that in this case the variability extends as far back as its Old French origin, espinage ~ (e)spinache.
So can we speak of a neutralization of voicing in the case of affricates in final position? No, because large and larch, edge and etch are always distinct. Well, can we say there is neutralization just in the case of final affricates in weak syllables, then? No, because Ipswich and so on never have dʒ, and because everyday words such as marriage, ˈmærɪdʒ, spillage, carnage, passage, porridge etc, not to mention Cambridge, never have tʃ.
The alternation is restricted to just these odd -wich proper names with a light first syllable (short vowel, one single following consonant), plus sandwich and spinach.