The literature surely helps. And with Greene and le Carre, it really is literature, not mere genre fiction. But what's also helped is the overestimation by its adversaries. Russia and Iran, for their own historical reasons, mythologize British intelligence to absurd degrees, and blame it for every real or perceived mishap. That no doubt helps with MI6's source recruitment of their nationals. Even the fuck-ups (Cambridge Five) somehow add to the mystique by becoming broader commentaries on British class and society.
On the rankings in general, it's rather silly to put Germany ahead of Poland and Estonia. KAPO, Estonia's internal security service, is the only shop to ever unmask a Russian spy, try, convict and imprison him, trade him back to Russia, then help him defect back to the nation that caught him. See following tweet for that yarn.
Poles are known by CIA for doing the "craziest shit imaginable," and CIA always means that in a good way. BND is on the mend, which is long overdue, but their last major (known) mole was outed by... GCHQ.
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