//If you study the original legislation, you will see that the companies had a 30 year "concession" to raise money, unless the money could be raised in less time. That 30 years is up this year. //
the Severn Bridges Act 1992, section 5 provided for the secretary of state (or his agent) to levy tolls until the power to do so expires, 35 years after "the appointed day". that would be some years from now.
the bridges revert to public ownership when Severn River Crossings plc have paid the DfT the amount prescribed by the act; this will happen this year, but the debt has accumulated to the point where £88M will still be outstanding, and this still needs to be funded by tolls. this has required separate legislation,
http://www.legislation.gov.uk/uksi/2017/1201/note/made which had the full backing of the Weslsh Assembly.