How old is English?
Did the Anglo-Saxons change all Brythonic place-names in the east? Changing place-names is a recent invention, and is merely applied in former colonies like in some African countries. We have a lot of Latin or Romanised place-names in England (e.g. Londinium) and we know that today’s names are often direct deformed descendants from the Roman ones (e.g. York = Civitas Eburacorum). Did the Anglo-Saxons only change the local proto-Welsh place-names, and left unchanged the Roman place-names? Very unlikely.
We also can argue
old place-names in France are, despite a heavy
latinisation of the
language, very well traceable to their original (para-)
Gildas: see sources
 Badonici montis = Bath?
there is some consensus amongst historians here. Some
but for a number of reasons the place Badbury is
An other question is whether Gildas would have known such a precise (and in fact redundant) information.
We know that Sulis was a goddess. Sulis is a genitive declension. The noun is Sule, Sules or simply Sul. The Romans associated her with Minerva . Minerva was the goddess of the mind: poetry, medicine, wisdom, commerce, crafts and combat skills. There is an English word for all this: the skills of the soul (Du: ziel, old low Franconian: sela, old Saxon: seola, old Fries: sele, old English: sawol, Gothic: saiwala.) The etymology is unknown, but it's clearly a proto-German word. There is a possible connection: Sule = soul. Bath was known as a healing place. Bathing in warm water was a luxury, and revived the soul. If Sule is the same as soul (maybe Sela in the local dialect), then this could be the proof that Bath was proto-English before the Romans conquered Britain. The local lord must have been a political ally to the Welsh in the 5th century.
 Early Wessex spanned over Oxfordshire and Berkshire. The West-Saxons called themselves the Wise-Saxons, the Wise Ones or (Latinized) Gewissae. They later expanded their power to the south and west. Eventually, in 1066, Harold, earl of Wessex, would become Harold, king of England.
Why Gildas  used the name Bath
The picture represents
the areas of
proto-English and proto-Welsh around 550 BC
'Bath' is a genuine Germanic word, which was imported into Welsh. We have no date for this, but it could have happened during or even before the Roman occupation. This can be the reason why the Welsh knew the city as Badon.
Bath could have been a Welsh place before the Romans conquered Britain. During the 5th century the city remained in the hands of the southwest Welsh (people who lived in modern Wiltshire, east Gloucestershire, Dorset, Somerset). The majority of the citizens in Bath however spoke proto-English.
The battle of Bath wasn't probably about
itself. In 577 the Saxons of Wessex  would win the
battle of Dyrham
and subsequently conquered the whole region (modern
a part of Wiltshire). They effectively separated the
Cornish people (or
southwest Welsh) from the Welsh-Welsh. Around 500, the
or Anglians (according to Bede) must have had the same
idea. At the
time Gildas wrote his sermon (around 544), the
proto-Welsh lords were
quarreling amongst themselves., thus creating an
for whom were in fact proto-English lords of Wessex.
Gildas knew that
Gloucestershire was the pivot of the Welsh defense. So
he urged the
'real Britons', the Brythonic speakers, to unite.