Based on Wikipedia bios – note that birthplace was just the most consistent information I could get. Where an actor is ‘from’ is a bit more complicated. Colin Baker is listed here under London but grew up in Rochdale – so really should be with the actors from the North of England.
Matt Smith is the outlier from the otherwise not unusual town of Northampton (OK it is also inherently unusual because Alan Moore is from there but aside from that…).
The clusters represent major ribbons of urban development in the UK: London obviously, but then the ribbon of towns and cities from Liverpool to Manchester to Leeds and then in Scotland the towns around and between Glasgow and Edinburgh.