This from Wikipedia,makes you think they were thinking of this as long ago as 2002.After all who would buy a bridge without a crossing charge,where would the profits be?
A fee was retained however under the new principle of Road User Charging, which had been introduced in the 2000 Transport Act as a means of using road pricing for controlling traffic congestion and funding local and national transport schemes. Under this act, the A282 Trunk Road (Dartford-Thurrock Crossing charging scheme) Order 2002 allowed the continuation of the crossing fee, which officially became a charge and not a toll on 1 April 2003. At the same time, under the terms of the 1988 Act the DRC company was liquidated and management of the crossing was contracted to Le Crossing Company Limited on behalf of the Highways Agency. Under the new user charge regime, Le Crossing collects the charges which are set in statutory Charging Orders under the 2000 Act, with the revenue passing in full to the government for redistribution, and annual public accounts of the operation published showing expenses/revenues of the crossing.