Sir George Cayley
Not quite so simple to say if its below 550 RVR its not CAT1
We now have OTS & LTS approaches (other than standard & lower than standard)
Good old Jersey R26 is a case in point the CAT 1 is 550m but it also has LTS CAT1 with a minima of 450m RVR the DA is unchanged.
R08 CAT 1 is 1000m RVR but just 600m on the LTS CAT 1
Not for the faint hearted given the runway length......