The producers of this show also did 'Immigration Street' , that was filmed in Southampton.
South of Watford covers a big area, all of Devon, Cornwall, Summerset. Plenty of parts of Surrey and Berkshire are safe as anywhere.
Talking Luton area to London Plymouth , B ristol ect are as far west as Hull is north
I see where your going, there's a ring around London which is in the larger part awful.
I don't mind getting a train into the West End for the evening, but I wouldn't give tuppence for the rest of it.