All very interesting in Euro/UK land, however...the original poster asked about a high transition layer situation, so...
The following is done in the USA where the transition layer is at FL180.
Climbing....leave altimeter on QNH until passing 17,500 feet, then change to 1013.2/29.92
Descending...leave altimeter on 1013.2/29.92 until passing FL180, then change to local QNH.
Japan is about the same, except the transition layer is FL140...as I recall.
Saudi Arabia....transition altitude 13,000 feet, transition level FL150.
Make altimeter setting changes at the specified point indicated climbing/descending.
Note.
In the above listed locations, this what ATC is
expecting you to be doing..airline SOP's or not.
Also, some might be wondering about the altitudes/flight levels assigned by ATC in these higher transition layer areas.
USA....normally, the lowest FL assigned is FL190, the highest altitude assigned is 17,000....however, in cases of very low pressures, these altitudes/Flight Level assignments (as indicated above) are changed to cater for the low pressure conditions...all done automatically by ATC.
Japan...same as USA, except with reference to the transition layer at FL140.
Saudi Arabia....
anything goes, beware