where does the money from the dartford toll go
It is used to cover the budget deficit.
Dartford Crossing
Subsequently, the chancellor George Osborne announced that charges would be increased instead to cover the budget deficit
wiki:5346037
Dartford Crossing [SEP] Charging scheme
After the change of government following the 2010 general election, the new prime minister David Cameron announced that the crossing might still be sold, despite local opposition, particularly from Gareth Johnson, member of parliament for Dartford. Subsequently, the chancellor George Osborne announced that charges would be increased instead to cover the budget deficit. Pre-pay accounts for the crossing were introduced around this time; drivers held an electronic device called a DART-Tag in the vehicle that automatically deducted the charge at payment booths. This was abolished when the Dart Charge was introduced in 2014. Under the 2008 Charging Order introduced on 15 November 2008, charges between 10 pm and 6 am were discontinued, but standard daytime rates increased, starting at £1.50 for cars.