For the moment I am starting with trying to identify such drifts:

#Goidelic k / c → #Brythonic p e.g. #Irish / #Scots ceann “head” vs Welsh pen
Brythonic gw‑ → Goidelic f‑ e.g. Welsh gwyn “white” vs Irish/Scots fionn
s‑ in Goidelic → h‑ in Brythonic sean “old” vs hen

I have to check

Iron age hoard found in North Yorkshire could change Britain’s history More than 800 objects unearthed near Melsonby show the north was ‘definitely not a backwater’ 2,000 years ago

When Yorkshire was less Ee Ba Gum! and more Duw Ffwc Aye!

#Brigantes #Brythonic #Welsh