A new mission for battlecruisers after the battle line accelerates

Are scouting-group battlecruisers still useful when the main battle line is already fast?

Yes, but in a different role: they become rapid-reaction, trade-protection, and commerce-warfare assets rather than scouts for the battle line.

Britain could base battle squadrons in the North Atlantic, Mediterranean, and Indian Ocean while holding very fast battlecruisers as a reinforcement force. Battlecruisers based at home or in the Mediterranean could sail to whichever station needed them. The same range and speed would support trade protection, interception, and commerce raiding. HMS Liverpool's interception of a Japanese liner to remove German passengers near Japan illustrates how an ordinary cruiser could carry out commerce warfare far from home. These missions give battlecruisers a place in the projected task-group fleet after their scouting role declines.