Each examinor who is testing at CPL and IR level is given there own call sign examXX. XX being a number. With the CAA chief examinor being EXAM01.
Some schools also with full permission of the authorities issue similar callsigns to thier instructors and the students use them when talking to ATC. When solo they prefix it with student or Solo depending on whats been agreed with the local ATS.
EXAM callsign is meant to mean you have a slightly higher priority than the normal one of zero compared to other light aircraft. And in general the controllers do make life easier for the aircraft.