At the time of the first contact with Rome the British or Brythonic form of the Celtic language was spoken throughout Britain

Closely related Celtic languages with only minor variations were spoken over large areas of western and central Europe. It has been suggested that peculiarities encountered in the names found in parts of Scotland are due to the influence of a pre-Celtic or even a non-Indo-European language. There is evidence that some names of natural features cannot have been coined by Celtic speakers but there are no names that are certainly non Indo-European.

The Latin imported by the Romans was the standard language

It was used by high status individuals found in the upper levels of the military and administrative heirachies. This language was also taught at schools set up to "Romanise" the sons of the British aristocracy. Vulgar spoken Latin was used by the rank and file of the army and this form will have had the most impact of the people at large.

With very few exceptions all names known in Britain are found in the works of classic Latin or Greek authors.

The exceptions are: the place-name HABITANCVM given in an inscription, the place-names Camulodunum, Verulamium and Calleva which are found on coins in the forms CA, CAM, CAMV, CAMVL, and CAMVLODVNO for Camulodunum, VER, VERL, VERO, VERLAMIO and VIR for Verulamium and CALLEV for Calleva.

The mass of names contained in the sources are to be interpreted as Latin names

They are based in the vast majority of cases on Celtic forms although only rarely can we recover the exact underlying form. The literary or official nature of the sources (and the fact that they were written for Greek or Latin speakers) probably precludes the use of contemporary spoken forms.