Somebody posted the relevant bit of the MoT tester's manual up there ^ , I welded up the sill on my daughter's 106 only a few weeks ago after it failed the first time round and it was accepted without question.
I'd go along with the other advice above though, make a good job of it before presenting it the first time and nobody will be any the wiser.
But this isnt about MOT's. Its about registering cars that have been welded.