100% apple. Flowers impossible for Betula which have catkins and wrong for Prunus. Could be genuine Malus sylvestrus or simply the result of a picknicker’s apple core. Along the sides of roads they’re often the latter. Motorists chuck them out and away they grow. If you go back in October you can assess the fruit.

Durr for Betula. Of course they have catkins. I was looking at the bark without engaging brain.

I suspect M.sylvestrus as there were 5, all on the same side of the road, and of different ages, but none of them young trees and all looked the same, apart from sizes.