The London (Heathrow) QNH is the one input in to the radar processors and is the one that drives what level readouts are seen on the radar displays for traffic operating below the TA.
However all the various approach units use their own QNH for traffic they are working at altitudes in their areas of responsibility.
The MATS Pt 2 states that any differences in the various QNHs used are deemed to be insignificant.
WF.