Neil will betray anyone: friends, colleagues, the Whitehall establishment,
whatever it takes for the "higher principle" of defeating the Russians.
Wellingham will let the Russians win a round, get away with some hideous
injustice, or whatever, for the "higher principle" of standing by his
friends, colleagues, and government and not breaking the rules.
Moral ambiguity is the hallmark of the show and one of the reasons it was
so great.
I'd much rather work for Wellingham -- he's not so likely to shoot you.
His moral failings are on the grand, international scale: choosing to
ignore some atrocity for the sake of political expediency.
By the way, with reference to Burnside's ambition: he may be ambitious, but
he always places "the good of the country" (the way HE defines it of course)
ahead of ambition. Time and again he does things that get him threatened
with the sack -- and he doesn't even do much to hide it. Anybody REALLY
ambitious would cover his tracks better, or not take the risk in the first
place.