The reasoning behind the rules and the way they are written
i.e. "the ground station may abbreviate the callsign" etc....
is that the controller is the only one who knows if there are two similar callsigns on frequency. Unless you have been listening out for some time before transmitting you will not be aware of other callsigns when you make your first call. On a frequency like London FIR you will not necessarily hear other callsigns transmitting on the same frequency if they are in another part of the country, so even if you do not hear similar callsigns you should not abbreviate your own until ATC has done it first.
You as the pilot of G-BRBA may well abbreviate your callsign on your second transmission but you do not know at that point if there is a G-OHBA on frequency. It is unlikely that there is, so normally you will not be corrected by the controller, but on the occasions where there is possibility for c/s confusion you can be sure that the reply from ATC will ask you to use all the letters.
's all common sense really.