The car slipped on a jack at some point, which is why the bottom of the rocker and castle are caved in upwards. This is very common problem in MGBs on any car that used the factory jack on ground that wasn’t completely flat and debris-free. The jacks were notorious for slipping and causing this damage.
As you can see, the inner sill has significantly more damage than on the other side. We’ll have to do a more comprehensive repair here.