That's a cracking job, and top marks for doing it from underneath. It is harder but it means you don't end up with an open seam for the water to get under. I probably wouldn't bother welding the inside either, just apply you're choice of primer / seam sealer / paint.
As stated, don't need to go mad grinding the welds down, I would usually just take the edge off, loose the sharp pointy bits, and clean it up, prime, paint etc, but you don't really need seem sealer over the weld on the outside anyway, so the tester should still be able to see the weld shape, maybe you'll want to put some normal underseal over it later.
It's an interesting question regarding visibility of weld to MOT tester, in theory it only applies if the tester failed it to start with. I mean if you did a full flush butt weld and ground the welds flat, you wouldn't be able to tell it had been welded.
P.s. Chris, you're on a warning !!