well, I don't think that it was specified that he went to London, it was only said that he fled away to England.
Anyway, Macduff suspected Macbeth as well, and decided to to go to England,where Malcolm is.
He gets welcomed by King Edward, where Malcolm is staying.
He tells him to come back and take the throne by force, and he asks King Edward to help them by providing some English armies.
Malcolm asks Macduff about his family to test his loyalty and see if Macduff is truly trustworthy. He pretends to be a tyrant like Macbeth to see if Macduff would still support him.
Malcolm advises Macduff to go out and fight to help place Malcolm on the throne. All Malcolm sees in Macduff's suffering is an opportunity to gain an ally.
Malcolm reveals his flaws to Macduff in order to test his loyalty and see if he truly has Scotland's best interests at heart. By pretending to be a worse candidate for king than he actually is, Malcolm is able to gauge Macduff's reaction and trustworthiness.
Malcolm initially mistrusts Macduff because of his close association with Macbeth. However, he later recognizes Macduff's loyalty and bravery, and comes to see him as a valuable ally in the fight against Macbeth.
Malcolm tells Macduff that if he had complete power, he would be even more tyrannical and corrupt than Macbeth, making him an unfit ruler. This is part of Malcolm's test to see if Macduff is truly loyal to Scotland and not just seeking power for himself.
Malcolm tells this to Macduff as a test to see if he, Macduff, is really loyal to Scotland or if he is there by request of Macbeth to retrieve Malcolm so Macbeth could kill him. This takes place when Macduff if trying to reclaim the crown of Scotland from Macbeth by force and put the rightful heir into throne. Before Macduff, many others had come to try and bring Malcolm back (or it is implied at least), so that is why Malcolm does not trust Macduff so he, like i said, tested him and his loyalty to Scotland
suspects Macduff may be trying to deceive him, as he has already been betrayed by those close to him. Malcolm tests Macduff's intentions by pretending to be worse than Macbeth, to see if Macduff is truly loyal to Scotland. Once satisfied of Macduff's loyalty, Malcolm reveals his true self and the two join forces to overthrow Macbeth.
He was testing Macduff to see if he genuinely had the interest of Scotland at heart. When Malcolm says "He hath not touched you yet" (IV, iii, 14) it means that he suspects Macduff is in league with Macbeth to take him prisoner and kill him. Malcolm then makes up all his vices to make sure that Macduff wants a good king and not just Malcolm. When Macduff responds that with his vices, Malcolm would make a terrible kind, Malcolm responds that he made the vices up and he knows that Macduff has Scotland's best interests at heart.
Malcolm is testing Macduff's integrity. If Macduff had embraced Malcolm's worldly, unethical responses, Malcolm would not have trusted him enough to form an alliance with him. When Macduff wails at the imminent hopelessness of Scotland, Malcolm knows he is in the presence of a good man, and he reveals his own "innocence" and stalwart loyalty to Scotland and to God.
Malcolm pretends to have all those vices to test Macduff's loyalty to Scotland and assess his true intentions. By accusing himself of vices, Malcolm is probing Macduff's loyalty to see if he is trustworthy and truly willing to fight for Scotland against Macbeth.
Macduff and Malcolm plan to overthrow Macbeth in England.
Both Malcolm and Macduff believe Macbeth is a tyrant to be overthrown. However, Malcolm is a tyrant himself, if not even more repulsive than Macbeth. Though Macduff is not fully aware of this fact. Malcolm manipulates Macduff into slaying Macbeth and then plea allegiance to himself as the King of Scotland.