Notch for the apron in the leg
I read a forum post of someone doing the English workbench build and they made a mistake I was about to make before I read the post. That was cutting the notch out for the apron on the legs. If your boards aren't "bloody big" boards like Richards's, then you have to adjust the size of your top rail. You need to make sure the top rail plus the width of the bearer is smaller than the width of your boards. If it is bigger you'll have a gap. It won't be seen but you'll know it is there!