at the point of WP change or before, that the weather was not fit for VFR ie less than 1nm horizontal vis and the ac was not 'clear of cloud, in sight of surface'?
Flipster, please note that your quote relates to CAA (civilian) VFR requirements. Military limits did not require as much as 1nm horizontal visibility. In RAFG we were required to operate VFR in something like 300 ft cloudbase, 500 metres visibility (whereas the meteorologists' definition of fog is visibility less than 1000 metres).
It's been a long time since I've had access to a copy; were the actual authorised weather limits for the flight (in VMC) laid down by the BOI report?