Even though the question was about TL in the UK, I feel the discussion did become a bit more general, and I therefore add some information about Norway;
Even though the pilot is given in a descend altitude at/below TA, the pilot shall not change altimeter setting to QNH before passing through the TL.
Same goes for departures, pilot shall remain on QNH untill passing TA climbing, even though he/she is cleared a flightlevel.
AIP Norway - ENR 1.7:
During climb to and when flying at the transition
altitude, the vertical position of the aircraft shall be
expressed in terms of altitudes.
After passing the transition altitude, aircraft during
climb shall change the altimeter setting to 1013.2
hPa, and thereafter the vertical position of the aircraft
shall be expressed in terms of flight levels.
Flights conducted at or above the transition level
shall express the vertical position of the aircraft in
terms of flight levels.
During descent, aircraft passing the transition level
shall change the altimeter setting to the current QNH
value for the aerodrome and thereafter express the
vertical position in terms of altitudes.
However, due to personal interest and my involvement in the Norwegian air traffic controllers association, I've discovered the following from the three main operators in Norway:
During descend;
- 2 operators set QNH upon receiving the QNH from the controller (and therefore a descend clearance below TA)
- 1 operator set QNH whenever they do their "descend-checklist", which might be in FL150, or FL100 or any level. (After receiving the QNH and a descend below TA from controller)
During climb;
- 2 operators set standard altimetersetting once they receive a clearance for a flightlevel,
- 1 operator fly on QNH until they climb through TA, or are above MSA (if higher) for the current airport, then changes to standard altimeter if received a clearance above TL
So one might wonder how this can happen, since their checklists are approved by the Norwegian CAA, and the same about the AIP..