I see more clearly now what you mean, and unfortunately I do not have specific data on British cemetery excavations. However I still think that you may get some interesting results from your non-metric dental traits in modern populations although the sample size and the comparing database are important to make a statistically significant comparison (for dental traits as for any other skeletal features).
I guess you may already have some of these texts, specially "The anthropology of modern human teeth. Dental morphology and its variation in recent human populations" by Scott and Turner. It is a very good comprehensive text about the traits and also about the conclusions that may be drawn about the data registered. It also has an extensive list with interesting eferences about dental studies in actual populations. There also other publications about discrete dental traits in modern populations that may be help you in case you want to do dental analysis in your sample such as: "Cladistic analysis of dental traits in recent humans using a fossil outgroup" by Stringer et al. (1997) JHE 32; "Characteristic High- and Low-Frequency Dental Traits in SubSaharan African Populations" by Irish (1997), Am.J. Phys.Anthrop. 102; "Dentition of Prehistorical Populations from Canary Islands: an Anthropological Study", JM Bermúdez de Castro PhD dissertation (1985); "Biological Affinties of Late Pleistocene Through Modern African Aboriginal Populations: The dental evidence". PhD Dissertation (1993). They illustrate consistent grouping of populations with different cultural and geographical origin.I do not really know if there are studies of this genie in British archaeological populations, but if your sample is large enough you may be able to draw some conclusions comparing with a proper outgroup.
Concerning those specimens with dental agenesia you mention, I recommend you to consult some medical and dentistry texts, because some alterations of the normal development of teeth like the distinct pattern of agenesia you comment (not all agenesia are developmentally the same) may have a chromosome defect-linked heritage -there are many different syndromes characterised by some agenesia and alterations, or framed in general osseous proliferation rate decrease with simplified morphologies affecting teeth and other skeletal elements. This would allow you to establish more reliable probabilities of familiar relationship, specially if found in the same burial and with possibility of associating it with other skeletal traits.
I hope this may be helpful for you.
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