Does this have anything to do with moving boundaries and were on the part of Chester's Deva Stadium that is in England?
South East England makes me think of Epping Forest, but it has no great height, although used today by mountain bikers. It has been used in lots of films, but I don't know if they had a Welsh connection. This is supposed to be a puzzle and is!
Anything to do with land which belonged to the Tudors ? Or to do with the origins of the word Welsh, which meant foreigner in anglo saxon.
Very very close, geographically speaking, Yorkie. The name of this place is believed to be one of the few in London derived from Welsh words.
Looking at Wiki it is all spelt out for us there. Never got near it. I was still going up and down Box Hill.
I was looking for all places in London beginning with something vaguely Celtic like Cam (Cym) etc. and got bogged down in Camberwell