having said that, it's nothing new, and both parties have done it. within (my) memory, james callaghan, john major, gordon brown and theresa may all took over as pm without a general election, as have others before them. (john major won - somewhat to everyone's surprise - a general election about 18 months later, and theresa may got the best of a draw about a year later.)
of course boris johnson is on record that he doesn't approve of people becoming PM "without a mandate from the british people" (more
here)