The CAA didn't like the perfectly acceptable solution of having the stick shaker operate at high speed (in place of RSB) as well as when approaching the stall.
The BA TriStars that went to the RAF (and became K1/KC1s) had RSB fitted (CAA certified). The Pan-Am ones (that became the C2/C2A) had the high speed shaker fitted (FAA certified). They were later brought into line and RSB fitted to the C2/C2A.
The CAA were concerned that at high altitude when a small margin existed between a Mach exceedance and the aerodynamic stall, the pilots would be confused as to the meaning of the stick shaker!
You are correct, Recovery Speed Brakes (RSB) were on CAA registered -500's only. Lockheed agreed with you and argued with the CAA that the stick shakers were more than effective and RSB was not required. However, the CAA would not accept Lockheed's data and RSB was added at the same time the extended wings/active controls were retrofitted.
Several years later, after BA sold their -500's to the RAF, Lockheed had to prepair a service bulletin for the two (2) Air Lanka -500's BA leased. The service bulletins incorporated the RSB computers and the addtion of vortex generators on the outer wing.