Up until the 1970s, British reggies were allocated in a strict sequence.
The AWNx, BIKx and BPEx sequences just came up in that order, although the BPEx jumped ahead a bit as homage to the old Guards Vans (G-APEx). BA also avoids having concurrent planes with same 'last two'. The CPEx came about because they forgot to reserve enough in the BPEx series.
When the CAA abandoned the sequential rule, BA joined in 'making up' reggies. 777s are obvious like you say, CIVx I guess IV=4(00) , DOCx I have no idea what this is supposed to convey. I believe you are correct in Kestrel's being people's initals.
All a bit anorakkie, but this stuff either interests one (like me) or not.