Callsigns were allocated sequentially and you could phone to find out where they were so you could pick something suitable that was coming up, you did not get your chosen callsign until they reached it though.I didn't get a choice for my callsign, but I guess that in the pre Internet days it wouldn't have been that easy to give that option, lol.
Worst case is you will fluff the exam and that's necessarily a bad thing - you will know what to expect when you do it next time.
Have you been on the RSGB site - they've got mock exam papers etc. there.
Ballcocks
Are you going to try again - you only have one area to brush up on so it seems a shame not to have another go.