The Roman Map of Britain R&C 19 next

First Interpretation Holcombe, Somerset
Milidunum (R&C 19) for *Meletonum?

A native fort, dunum, on the River Mells could explain Milidunum. One is present on Blacker's Hill (ST 636 500) near the river's source. A few miles eastward on the opposite side of Margary 5b (Foss Way. Ilchester-Bath) is the Holcombe Villa (ST6750), also on the Mells, where the name might have been transferred. Blacker's Hill is an Iron Age hillfort. 
Both Milidunum and Meletio share M-l-d/t. The Afon Mellte of Breconshire is recorded in LL as Melltou. W. mellte 'lightning'. 

Second interpretation  Rangeworthy, Avon
Milidunum (R&C 19) for *Litanum or*Lidanum

Ekwall does not equate Ladden Brook (the upper part of the Frome) with the Leadon, Lidden and Lodden of Gloucestershire and Dorset. Those he derives from British litano 'broad, extensive', Welsh llydan, lledan, Breton ledan; also Irish leathan. 'Extensive' accurately describes the many branches of Ladden Brook. I do not know if 'broad' is appropriate.

The Roman settlement at Rangeworthy (ST7085) is on Ladden Brook and the Roman road Margary 541a (Gloucester - Willsbridge, which becomes Margary 540 on crossing the Avon). (At the time these roads were numbered the section beyond Engine Common connecting to Willsbridge was unknown.)