Milton is celebrated for his defence of free speech (within certain limits, of course – Catholics excluded!) and freedom of the press. (See his Areopagitica.)
From the little I know of Sir Francis Bacon (Viscount St Albans), I’m not at all sure what contributions to the liberal tradition can be attributed to him. I have always regarded him as a self-serving time-server and, in his rivalry with Coke, an enemy of the common law and a supporter of the royal prerogative. His scientific/philosophical work is moderately interesting though I believe it to be overrated.
Cato the Younger was a resolute opponent of the new Roman politics that develop in the first century BC. He worked with Cicero to prosecute Cataline and he opposed Caesar and the triumvirate. But, as was the case with Cicero, the drift of Rome from republicanism to embryonic empire was pretty much unstoppable and despite his best efforts, the Caesarean faction, in the person of Augustus, finally won out. Together with Cicero, he is one of my heroes.