In a minute I will search for the video on youtube where that claim is proposed and argued. Now the argument is basically this: on such and such a place, five placenames end on -ey, that is Ænglisc for island. However, there were no islands but only hills after a lake was dried out, which it was either before Romans came or by their skill, because they left a Roman road there before the Anglo-Saxon invasion. So, the places have kept their names since the Romans or since before the Romans (during the Roman rule, no Germanic tribes were imported), and the names are English names ... ergo a language related to English was spoken in that area before the Romans came.
Now, I agree the places have kept their names since pre-Roman times, I agree that the names as now extant are in English, that is in a Germanic tongue. But it does not follow that the names were originally so. I think these names were originally Celtic and included the word Inis, or rather, since Inis is the Irish form, the Ancient British and perhaps also Old Welsh cognate of Inis. And when these people changed language from Celtic to English, they translated the names as they were, irrespectively of the fact that these "Inis" places were no longer islands./HGL
Video on Proto-English Theory: Cerney, Oaksey, Minety, Ampney, Meysey and other "English placenames that must have been there before the Romans" (i e placenames that now are English and Germanic and were there before the Romans or early in their stay)
PS: I would of course argue that Celtic - more or less Romanised - was spoken both by the "East British" and the "West British" varieties of Homo Europaeus. That in turn would indicate that their earlier or later adoption of English and their preference for Roman versus Celtic Church (which is where Belloc found the divide between courts speaking Welsh and courts speaking English) did have some connexion, either with fine shades of race affinity, or, more probably, with same factors of the terrain that these race affinities also reflect./HGL
PPS: While we are at Belloc, as said he thought that Old English spread with the Roman rite of Christianity, whereas a court speaking some British lingo was adhering to the Culdee monks rather than those of Saint Bennett, to bishops under abbots rather than bishops under Rome./HGL