We do have it John on many bridges and regular users have them and have special booths for their use. All these people who make these modern concepts assume everyone has a modern phone and is digit happy, they don't think that not every driver has a computer at home, or has access to a payzone shop, in London these shops are on Red Routes no stopping allowed at any time, similar systems in other towns also.
All tolls on the Severn bridge are to be discontinued at the end of 2019, but of course they could still change their minds