Well, I'd assume it was one of Boris or JRM - ie Boris, I suspect JRM is smart enough to avoid top office. There's about 50 MPs in the ERG. This leaves..about 250 odd Tories to nominate someone else as the Not Boris candidate.
Now, do we know anyone who really wants to be PM, who has a habit of writing newspaper columns where he tries to look reasonable instead of batshit, who has a history of antagonizing Boris Johnson, who has spent the last year buddying up to May in the hope her supporters will support him in any successor's play off, and, most crucially, who is massively popular with the Tory grassroots, for some reason. And isn't Boris Johnson, so likely to win any Anybody But Boris votes in a 2 person playoff ala Major over Heseltine, Thatcher over Heath, etc etc.
All of which tells me our next Prime Minister will be this guy: