Going back to the start of this thread. The 18-30 Rule is actually enshrined in CAP371 A copy of which i have in front of me. As in Airclues' post about BA Rules, it reduces the allowable flying duty period if not acclimatised to local time. Other than this, the official line summarised from para 2.3. Is that scheduling rest periods of between 18-30 Hours is an undesirable practice & is a factor to be considered when planning rosters.
So that, for the record, is the official line.