How many people drive through that tunnel without prior thought or route planning, very few I would say unless problems at Blackwall Tunnel are reported, so it is not a problem to prepay.
I would think there will be many heading south to Dover who wont pay and just drive straight through...
Would assume that all vehicles will be subject to cameras and number plate recognition to ensure payment. Does that more foreigners will go to Dover owing money. Why they cannot link fines etc to departure computers at ports etc to ensure nobody leaves owing money I dont know.