Remain outside CAS has been standard in the UK since the 1970s at least, and was introduced because of pilots who believed that the fact that they were talking to the ATC unit believed that they had a right to enter CAS, whether a clearance had been issued or not. This led to as high a number of airspace busts as we have today, and as a defensive measure the phrase was introduced as a barrier. In the old FIR positions, when aircraft requested joining or crossing clearances of airways, a time check was also added, so that if the sector issued a clearance such as 'cross XXX not before TIME' everyone was singing from the same hymn sheet. It is a barrier, just like we use 'after departure' now in amended clearances, rather than 'after take off', because historically aircraft, on hearing an amended clearance phrased in the former manner, lined up without permission. It could be still considered a useful barrier.
HB