Channel Express had two H.P. Heralds named after the Engineering Directors sons: G-SCTT and G-STVN.
Later there was a competition among the staff to choose the registration for a F27 that came out as G-CHNX. My suggestion that the company reserve the G-CEX..x series did not win but that seemed to be the way forward for a few years until the arrival of the Boeing 737s.
So to answer the OPs question: Various people come up with the choice of registration but the senior management will make the ultimate decision.