[robin2]

My guess is that this might be cycling related; that an area where there's likely to be public-spirited cyclists with GPS devices is well placed to get good OSM coverage.

In support of my theory, I point to Cambridge being a little bit of a hotspot in the southeast of England. (Not so much as Clacton though ... hmmm ... maybe the theory needs some work.)

[Brakenshire]

Isn't Cambridge where OpenStreetMap started? That's a bit of a confounding factor!

[dbaupp]

I believe Cambridge was the first place to be declared "complete".

[ris]

"Isn't Cambridge where OpenStreetMap started?"

No. London.

[SteveCoast]

Well, kinda. A lot of the coding and early work was done in Cambridge but the first node I think was in Regents Park.