If like me your from north of Watford the number of times you cross the QE2 bridge or via the tunnel at Dartford is only when heading down / back to/ from Dover for the ferry or EuroTunnel at Folkestone once or twice a year.

Previously there were payment cabins at the south side but as of October last year you need either to pre-pay or pay on-line within 24hrs for if not they will impose a fine of £70.
To date I believe 350,000 fines have been issued since last October.
The cost for a car and caravan is £2.50.
You can set-up a prepaid account or pay after crossing at the below website.
https://www.gov.uk/pay-dartford-crossing-charge

I've put £5 in to cover my trip over and back in June.

David