Who is Dave Horwill? Don't you mean the late Frank Horwill?
However it was George Gandy who was actually the coach at Loughborough (or so I understand).
Coe was more of a 1500/3000m runner in HS (according to the Coe/Martin book), running 3.45 and 8.14 as a Junior (age-group). He doesn't appear in the top 25 UK as a Junior over 800m, so likely a combination of late maturation (5'9", 119lbs at 19yo) and a good strength development program at Loughborough helped move him toward faster times over 400m/800m.
It is also possible that it took him years to adapt to the program of his father as they seemed to experiment with a number of things that appear to have been over-doing it a bit (Horwill's influence) at times. He lost a year to stress fractures at 18yo as well.
It does show what a talented, motivated athlete can do, even with no "real" coach - his father had no formal training as a coach - but Seb was an "experiment" that worked out well in the long run, as Coach Coe was prepared to learn from others (so kept an open mind), which too many other coaches don't seem to do - it's their way or the highway (even if their way is wrong for a given individual).