The idea is to follow an existing French model for cheaper intercity travel that cuts costs by using older tracks and suburban terminuses. So far, research into a cheaper service has focused on if and how it might be feasible, rather than exactly when—but has found that such a service is eminently implementable and viable along the lines of companies already in existence.
https://www.citylab.com/transportation/2018/07/cheap-train-tickets-london-paris-getlink-channel-tunnel/564530/
https://www.citylab.com/transportation/2018/07/cheap-train-tickets-london-paris-getlink-channel-tunnel/564530/