My own suspicion is that the timing of the resignation is no accident. IMMEDIATELY AFTER STEELE AGREES TO COOPERATE WITH SENATE INVESTIGATION, this insect resigns?
Steele was connected to government at the highest levels in both England and the U.S. so it's not beyond possibility that it was the ambassador's people themselves who leaked his hateful anti-Trump memos to give him cover for the departure, which gives him the ability to avoid being subjected to the requirements of British law governing public servants. IOW, he may have less obligation to cooperate with any probe into misconduct by British officials.
Maybe I've grown too suspicious over the years, but it is my first impulse to assume that leftists NEVER do ANYTHING unless it is in their own personal best interests above and beyond everything else.