22/04
In answer to your query, I flew in the flight checking role in the RAF for years.
The acceptable tolerances were so small that even when an ILS failed it's calibration I don't ever remember feeling that a safe landing could not have been made from decision height, even in the most marginal of conditions.
Certainly, the aircraft would never have been more than a few feet off the centreline and would also be safely in the touchdown zone, even with a 'failed' ILS.