Mark Carney will be leaving at the end of January 2020
The decision to name a successor to current BOE Governor Mark Carney until after Brexit (on October 31, 2019).
Carney has postponed his leaving twice already but has said the end of January 2020 will be his final day at the helm. So there will still be some time to transition a new leader.
The BOE Governor does not have to be from England. In fact, Mark Carney is Canadian and was also a former head of the Bank of Canada (from 2008 to 2013 when he took the role of the BOE head).
The former Chancellor of the Exchequer Philip Hammond said he’s willing to look abroad for a central bank head, emphasizing the need for a replacement with international stature. As far as I know, Sajid Javid has not said one way or the other, but there is nothing stopping him for looking outside the UK.