[rmc]

The nationalistic Welsh will tell you they are the real british people, but displaced and pushed westwards (into what is now Wales) by the Romans, Normans, Anglo Saxons etc. So a name might be mostly Welsh now, but the people with that name might have lived in that area for a very long time,

[grifaton]

I'm not sure about this - first-century London was about the size of Hyde Park!

http://en.wikipedia.org/wiki/Roman_London#First_century_AD

Edit - also, it appears that prior to the fifteenth century, the Welsh tended to use a patronymic naming system.

http://en.wikipedia.org/wiki/Welsh_surnames