Tim Cook said the words himself out of his very own mouth when introducing the SE3, that the SE line is for people who like smaller iphones. The last time I checked, the Mini is considerably smaller than the XR/11. Also, a XR/11 with bezels so thick in order to shave off 0.4 inches of screen space to go from 6.1" to 5.7" is just ludicrous.
Personally though, i don't think the next SE will be the Mini design, its too early to sell that design at £429, the next small iphone in the pot is the X,XS,11 Pro 5.8" chassis.