I know that James Slack was the last PMOS under Theresa May. Who is in this position under Boris Johnson?
As of 26 July 2019, it seems James Slack is Johnson's spokesman, at least for now. From a Bloomberg article timestamped July 26, 2019, 7:17 PM GMT+2:
After the EU rejected the demands, Johnson’s spokesman, James Slack, suggested there’s little point to talks unless the bloc accepts the U.K. position.