Well.....
The Basingstoke has the nickname 'Doughnut City'!, and is (apparently) officially the most boring place in the UK due to the high number of accountants who live there! I found it funny that, after Swindon, Basingstoke has the most roundabouts in the country!!!! Who counted them?!:rotfl:
As for Dartford, I've found zilch!

Apart from the football team's nickname - 'The Darts'. :sorry:
Most nicknames are regional or derogatorily given by neighbouring (rival) towns, which is why rural villagers are usually 'Woolly-Backs' or worse!
Keep them coming...........