Keithl: thanks for that. ATC are required to pass the initial RTF message, stating that the RVR is below the absolute minima for that particular approach and requesting your intentions when you declare an intention to commence the approach (defined as first accepting vectors for an instrument approach or leaving the initial approach/holding fix) and the RVR is below the absloute minima.
So, as an example, if the absolute minima for the Cat 1 ILS is, say, 700m RVR and you declare an intention to commence the approach with 600m RVR at that time, you should receive the spiel. If you then confirm that it is your intention to continue with the approach, you get the second RTF message, re. possible contravening UK legislation etc.