Pardon offers are in no way binding or enforceable, so no actual gambling is involved. You just don't follow through unless you get the answer you want, as well as making very clear (such as by spending multiple years building a particular public narrative which the defendant actively participated in around your preferred answer) what the expected answer is so that, regardless of what the true answer is, the defendant knows what answer gets them free of the charges.
This is in a British court, and the President is not empowered to gag the courts anyway (except sort of I guess, at the highly-constitutionally-questionable national security trials)