"As I understand it, the internal pressures are too high to allow actual spark production until there is a flame-out - then they start to work properly. Do you know if this is correct?"
Sorry, Cornish...
My training really didn't go that deeply into this kind of stuff. If a car's spark plug doesn't have any difficulties discharging in a combustion chamber, I wouldn't have thought that a 10 joule spark would have had any difficulties(???).
According to my notes, at takeoff thrust, the max px in an RB211 core is a mere 425psia. Perhaps it's not the spark that is the problem here, but the way the fuel reacts with the spark at these pressures/air velocities (just guessing here!).
Cheers.
Q.