The former BOAC (Speedbird c/s) and BEA (Bealine c/s) operations were merged in 1974 using Speedbird, and the following year the enhanced domestic operations of the no-reservations BA Shuttle were started. There were just too many chances of confusion in RT calls around London (and possibly some incidents) so the domestic flights were given a new callsign of Shuttle.
The Shuttle concept was eventually abandoned and the services reverted to a normal scheduled operation, but the callsign remains, and is employed on other domestic routes as well which were never part of the proper Shuttle operation.