Examples of Ancient Brythonic words in modern English?

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[citation needed]. 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.

There are many words in modern Welsh that have their roots in the Celtic/Romano common language (something almost equivalent to what we see now with pidgin English). For example:

• The Welsh for window is ffenest.
• Day is diwrnod (same Latin root as diurnal).
• Fish is pysgodyn (same Latin root as pisces and piscatorial) and fishmonger is gwerthwr pysgod.
• Church is eglwys (from the Latin ecclesia)
• Bridge is either pont (Latin: pons) or bont depending on the word placed before (linguistic phenomenon called "mutation"). It appears in place names such as Tal-y-bont and Pontardawe.

The Brythonic word U̯entā means favoured/chosen, so I would assume Derwent means "favoured water"

And the welsh for White is gwyn not gwent and is derived from Brythonic U̯īndos which means white.