@ BOAC:
Its stated in FCOM Vol 3, Supplementary Techniques, Flight Controls, Flight Mode. (3.04.27 P3).
I think you understand but for the ones which do not:
Pls don't consider the THS being locked but read as:
"If THS > 8 up (and no autotrim available), full elevator pitch down authority may be insufficient for speeds above 180 knots."
This also from FCOM 3 and mentioned several times in the 'Abnormal and Emergency' section.