Since, in Arthur's day there was no such place as England, & 'Arthur' fought one or two battles up among the Strathclyde folks, I assume you are meaning Britain when you say "England"? Moreover, any Welsh word then would refer not to "England" but to something else. Pesumably, the local people he was doing battle with in Britain were Anglo-Saxons, Scots (Irish), Picts, Brythonic Celts & Romano Britons. Would the Welsh word quoted not be a reference to anything alien to them.

The oldest settlement believed on the Thames was Belan's Gate, c400BC.,(Billingsgate) until quite recently. Now a more ancient bridge remnant has been found dated at 2000 BC, I believe.
Bill Charlton