In just about any language, place names are the oldest words, and are often taken over from the indigenous people from whom the land was taken. So place names in England are going to be your best bet there.
Here's what wikipedia had to say about it:
The principal legacy left behind in those territories from which the
Brythonic languages were displaced is that of toponyms (place names)
and hydronyms (river names). There are many Brythonic place names in
lowland Scotland and in the parts of England where it is agreed that
substantial Brythonic speakers remained (Brythonic names, apart from
those of the former Romano-British towns, are scarce over most of
England). Names derived (sometimes indirectly) from Brythonic include
London, Penicuik, Perth, Aberdeen, York, Dorchester, Dover and
Colchester. Brythonic elements found in England
include bre- and bal- for hills, and carr for a high rocky place,
while some such as combe or coomb(e) for a small deep valley and tor
for a hill are examples of Brythonic words that were borrowed into
English. Others reflect the presence of Brythons, such as Dumbarton –
from the Scottish Gaelic Dùn Breatainn meaning "Fort of the Britons",
or Walton (several) meaning a 'tun' or settlement where 'walha'
(Welsh/Brythons) still lived.
The number of Celtic river names in England generally increases from
east to west, a map showing these being given by Jackson. These names
include ones such as Avon, Chew, Frome, Axe, Brue and Exe. Also river
names containing the roots " der- / dar- / dur- " and " -went " E.G. "
Derwent, Darwen,Dart,Deer, Adur, Dour,Darent, Went ". The Celtic
origins seem likely, the meanings more controversial: Some associate "
Der- / Dar- " with the Brythonic word for " OAK(S) " ( " derv /
dervenn" in Breton, " derow / derowenn " in Cornish " derw / derwen "
in Welsh. Possible but there would have been a lot of oaks around;
maybe there was. As to " -went " some claim this to be a word for "
valley " or associated with the Celtic word " nant " for river ( like
in Welsh ). This seems a very unlikely derivation, as there is no
known initial consonantal changes from " n- " to " w- ". More likely
is that the " Der- / Dar- / Dur- " means " water " [ c.f. " Dour " in
Breton, dowr in Cornish, Dŵr in Welsh.] and " -(g)wen(n)(t) " means
white / pure.