From the old AFTN designators; Denmark uses EK because ED was already allocated to West Germany. ( East germany had ET IIRC) EK fits nicely with Køpenhagen.
In the UK the third letter designated the circuit therefore GW for Luton and GD for bristol Lulsgate and for most (all?) civilian airfields in Scotland having P, Q for the military
Of course, this was all still in the future when I started.
