(hint: it's almost certainly a Labrador Retriever)
(that's the dog at top left below):
The BBC has analysed 10 years of microchipping data & put it up as an interactive-postcode map of Britain.
One surprising result:- there are *far* more dogs in Notts than Herts (9,800 Labrador Retrievers, etc. in 'Nottingham' but just 4,500 in Stevenage).