In PN BdHu:30 it is stated that Carlton in Bedfordshire is the most southerly example of this Scandinavian place-name type.   I doubt that Mawer and Stenton did a precise check against Carlton near Saxmundham in Suffolk, for it is a very close race.   Taking the churches of the respective villages as reference points, I make the grid references and latitudes to be:

Carlton in Bedfordshire SP954552 52.186
Carlton in Suffolk            TM382640 52.222

Thus Carlton in Bedfordshire just wins by about 2 minutes.

However, the true winner is likely to be a lost Carlton in Carlford Hundred in Suffolk.  Carlford Hundred lies completely to the south of either Cartlon above.   It is Calletuna (sic) DB, Carletuna 1086 (c.1180) InqEl, probably Gilbertus de Carleton 1327 SR.   There is a caveat that it might be from a personal name Karli.  Cf. Anderson, English hundred-names i.92.

Keith