The area where the inner wing, bulkhead/toeboard and sill meet is highly stressed, and on that basis the MOT tester would be entitled to fail it, because it could be unsafe to drive, same as for rust in the middle of a sill, even if it was more than 300mmm from an important mount. In any case it is almost certainly within 300mm of the rear mount of the front subframe, and maybe the damper mount.
Obviously, a good patch weld job will resolve the problem for now, and it will last a lot longer if the rusty area is cut out and new metal butt welded in, followed by proper preparation and paint.
Contrary to what has been said, the front wings are structural, as they brace the inner wing, which carries subframe loads. However, the patch of corrosion does not impair the structural performance seriously, although it will likely be impossible to prevent it spreading without a new wing in the long term.
But, as in all Mini bodywork problems, there is usually more lurking, out of sight. Probably one of the main reasons why I bought a MIG welder many years ago....