Raising the TA won't be a bad thing but if it is 18k what QNH would that be based on and would approach units then have to change the aircraft to the airfield QNH later in the approach phase or would QFE make a comeback?. If so what level would that be done?
I Imagine it would be ok as the disparity between say a london TMA QNH and say the birmingham airfield measured QNH would not be as big as you sometimes get between QNE and QNH.
for non uk
QNE = altimeter setting for flight level
QNH= altitude (above sea level)
QFE = Height above aerodrome elevation