It allowed them to intercept ships to check if they were slave trading. While they were inspecting the ship, the British officers could use the pretext to find other things wrong, ie smuggling, piracy, desertion, ect. Then they could seize your property or arrest you in some cases. Sort of like how American cops can use a busted taillight to pull you over, then arrest you if he find drugs during that.
The navy did also free a significant number of slaves this way too so it wasn't just an abuse of power, but it also signaled to everyone that Britain was the dominant naval power in Europe now.
Probably was Aldrich Aims, he's is one of the best known American double agents and passed a bunch of lie detectors. He did it for money though so he's not as cool as the Cambridge Five in my opinion.
Kendall Myers is his wife Gwendolyn were very based however. They worked for the state department and passed secrets to the Cuban government for 30 years out of ideological loyalty.
Unfortunately they caught him and he is now locked up at ADX Florence supermax facility, where they put all the terrorists.