G -- reclassification of the car to a different IT category is treated differently from a PCA. It is basically a recognition that the car was classed wrong in the first place, and is specifically allowed by the GCR. Quite a few 2.0 16v cars have been moved from S to A, correctly, in the last few years.

Is the MGB GT in B? It is probably a C car, maybe that is the correct route for you to go if a 10/10 MGB GT doesn't run well with the Volvos and Golfs and Chargers and Audis in B.